Validate Support for MySQL 5.7

[altmannmarcelo]

Description

A lot of people are still using MySQL 5.7. We should check if we currently support MySQL 5.7.

Main area to check is around Snapshoting as LOCK INSTANCE FOR BACKUP is not available.

Change in user-visible behavior

Requires documentation change

[altmannmarcelo]

Another place to adjust:

const CHECKSUM_QUERY: &str = "SET @source_binlog_checksum='CRC32'";

This should be conditionally changed to master_binlog_checksum if we are in 5.7, same as we do for SHOW MASTER STATUS;