Add command for purging avrorouter files and state

A command similar to RESET MASTER [TO <file>] that allows the resetting of the conversion state to some specific binlog would allow resetting of the avrorouter without manually deleting files.

This will significantly speed up as well as simplify testing. In addition to this, it will make maintenance operations slightly easier when old Avro files are automatically purged and the state reset in a consistent fashion.