implement --max_parallel_snapshot_tables for MySQL

**Description**

We currently hard code `const MAX_SNAPSHOT_BATCH: usize = 8;` on MySQL. 

A config `max_parallel_snapshot_tables` exists to allow change that. We should plumb this to mysql.

Also, add an ENV option for `max_parallel_snapshot_tables`

**Change in user-visible behavior**

**Requires documentation change**