ldb key_value: Add batch_mode option

When performing a join the overhead of the sub transactions protecting
key value operations becomes significant.  This commit adds a new
"batch_mode" option that disables the sub transactions around key value
operations.

The operation level index cache is also disabled, which means the
overall transaction level index cache can become inconsistent if an
operation fails. To protect against this and other possible on disk
inconsistencies, if any operation fails during a batch_mode
transaction the commit will fail and transaction will be rolled back.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
