MySQL `source` field in change events has binlog position for next event

The position that we’re recording in the offsets is actually the position of the next event in the binlog. This makes it easy to restart, since the last offset we recorded has the position in the binlog where we’re supposed to start. But we’re also including that position in the `source` field, which IMO is semantically wrong.

The `source` field should reference the binlog coordinates where that event appeared. See the forum discussion in which everyone agreed that it should be fixed.