MySQL: Improve parallel snapshot

[Amogh-Bharadwaj]

1. Fix comparison of previous partition with current partition.
2. Use a different query to obtain logical partitions for MySQL

• Adapt query to be able to handle timestamps

[iskakaushik]

I think we should be able to unit test this better. One way to do it would be to create two helper structs:

PostgresPartitionedQueryGenerator and MySql.... feed them a column name and data type and assert on the generated query.

[heavycrystal]

are there benchmarks for how much of a speedup this led to? would like to see those attached