Possible data corruption using HD5Store
I noticed some really weird behaviour of new HD5Store. I am using pandas 0.10.0 and pytables 2.4.0.

When I try to serialize my Panel structure and then remove it from the store, the memory is not reclaimed - the store appears to be empty while it is still using 90MB on my hard drive.

I am trying to find any repro steps using synthetic data but so far I am failing - It is working as I would expect (file becomes empty after removal of the panel). I can't give you the actual files/data but maybe any stats of the h5 file itself would be helpful.

What makes it even worse, when I repeat the same operation over and over again, h5 file grows to ~num_iterations \* 90MB in size while still appearing as empty. I am able to reproduce it every single time with my dataset.

My code:

``` python
# some_panel is a ~ 30 x 10000 x 30 structure with mostly floats:
store = pd.HDFStore(file_name)
store["panel"] = some_panel
del store["panel"]
store
```

Output:

```
<class 'pandas.io.pytables.HDFStore'>
File path: <my_file_name>
Empty
```

However, the file is 90M big. Here is the h5dump output of my file:

```
HDF5 "<my_file_name.h5>" {
GROUP "/" {
   ATTRIBUTE "CLASS" {
      DATATYPE  H5T_STRING {
         STRSIZE 5;
         STRPAD H5T_STR_NULLTERM;
         CSET H5T_CSET_ASCII;
         CTYPE H5T_C_S1;
      }
      DATASPACE  SCALAR
      DATA {
      (0): "GROUP"
      }
   }
   ATTRIBUTE "PYTABLES_FORMAT_VERSION" {
      DATATYPE  H5T_STRING {
         STRSIZE 3;
         STRPAD H5T_STR_NULLTERM;
         CSET H5T_CSET_ASCII;
         CTYPE H5T_C_S1;
      }
      DATASPACE  SCALAR
      DATA {
      (0): "2.0"
      }
   }
   ATTRIBUTE "TITLE" {
      DATATYPE  H5T_STRING {
         STRSIZE 1;
         STRPAD H5T_STR_NULLTERM;
         CSET H5T_CSET_ASCII;
         CTYPE H5T_C_S1;
      }
      DATASPACE  SCALAR
      DATA {
      (0): ""
      }
   }
   ATTRIBUTE "VERSION" {
      DATATYPE  H5T_STRING {
         STRSIZE 3;
         STRPAD H5T_STR_NULLTERM;
         CSET H5T_CSET_ASCII;
         CTYPE H5T_C_S1;
      }
      DATASPACE  SCALAR
      DATA {
      (0): "1.0"
      }
   }
}
}
```

and h5debug output:

```
Reading signature at address 0 (rel)
File Super Block...
File name (as opened):                             <my_file_name>
File name (after resolving symlinks):             <my_file_name>
File access flags                                  0x00000000
File open reference count:                         1
Address of super block:                            0 (abs)
Size of userblock:                                 0 bytes
Superblock version number:                         0
Free list version number:                          0
Root group symbol table entry version number:      0
Shared header version number:                      0
Size of file offsets (haddr_t type):               8 bytes
Size of file lengths (hsize_t type):               8 bytes
Symbol table leaf node 1/2 rank:                   4
Symbol table internal node 1/2 rank:               16
File status flags:                                 0x00
Superblock extension address:                      UNDEF (rel)
Shared object header message table address:        UNDEF (rel)
Shared object header message version number:       0
Number of shared object header message indexes:    0
Address of driver information block:               UNDEF (rel)
Root group symbol table entry:
   Name offset into private heap:                  0
   Object header address:                          96
   Cache info type:                                Symbol Table
   Cached entry information:
      B-tree address:                              136
      Heap address:                                680
```

and h5stat:

```
Filename: <my_file_name>
File information
        # of unique groups: 1
        # of unique datasets: 0
        # of unique named datatypes: 0
        # of unique links: 0
        # of unique other: 0
        Max. # of links to object: 1
        Max. # of objects in group: 0
File space information for file metadata (in bytes):
        Superblock extension: 0
        User block: 0
        Object headers: (total/unused)
                Groups: 272/0
                Datasets(exclude compact data): 0/0
                Datatypes: 0/0
        Groups:
                B-tree/List: 544
                Heap: 120
        Attributes:
                B-tree/List: 0
                Heap: 0
        Chunked datasets:
                Index: 0
        Datasets:
                Heap: 0
        Shared Messages:
                Header: 0
                B-tree/List: 0
                Heap: 0
Small groups:
        # of groups of size 0: 1
        Total # of small groups: 1
Group bins:
        # of groups of size 0: 1
        Total # of groups: 1
Small # of attributes:
        # of objects with 4 attributes: 1
        Total # of objects with small # of attributes: 1
Attribute bins:
        # of objects with 1 - 9 attributes: 1
        Total # of objects with attributes: 1
        Max. # of attributes to objects: 4
Summary of file space information:
  File metadata: 936 bytes
  Raw data: 0 bytes
  Unaccounted space: 94094088 bytes
Total space: 94095024 bytes
```

Total space is showing actual space on the drive ~90MB. Let me know if I can provide any more information
