Mongo cursor cleanup

Currently the mongo connector opens a cursor to the oplog without timeout: https://github.com/debezium/debezium/blob/master/debezium-connector-mongodb/src/main/java/io/debezium/connector/mongodb/Replicator.java#L402

This cursor does not get closed on task shutdown and since there's no timeout mongo itself will not close it ever too, thus we see ever increasing cursor counts.

Proper closing of the cursor only solves the problem partially as task crashes will leave hanging cursors indefinitely, so maybe a timeout is needed after all ?