"snapshot" attribute should be false instead of null for events based on the binlog

The source metadata block of change messages has a snapshot attribute, which has the value true for change events emitted during snapshotting. For events emitted during binlog reading its null, though, whereas it'd be more reasonable to be set to false.

We should:

• double-check with the docs whether there are promises that the property is null or whether we're free to set it to false (which I'd expect)
• check whether it's an issue with the MySQL connector only or the others, too