BENCH: add benchmarks for SQL
xref #6701, #6416

WIP for now: intitial benchmarks for sql functions
- Where should the benches be placed? Because there is `io_bench.py`,  `packers.py` and `parsers_vb.py` which all contain io related benchmarks. Or should I create a new file?
- The first tests are a generic mixed-type frame writing/reading, both with sqlalchemy (sqlite) and sqlite fallback
  - I am going to add seperate benches for different types (float, string, datetime)
  - other important things to test?

First run with `./test_perf.sh -b master -t HEAD -r sql` gives:

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
sql_write_fallback                           | 218.5094 | 219.5977 |   0.9950 |
sql_write_sqlalchemy                         | 347.1383 | 347.9416 |   0.9977 |
sql_read_query_fallback                      |  28.5973 |  28.6067 |   0.9997 |
sql_read_table_sqlalchemy                    |  39.0046 |  38.9366 |   1.0017 |
sql_read_query_fallback                      |  35.9294 |  35.4296 |   1.0141 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [3bcc9c2] : BENCH: add benchmarks for SQL
Base   [86ecb99] : BUG: fix iat and at for Float64Index
```

You can see that the sqlalchemy version is somewhat slower than the fallback version.
