MongoDB connector should periodically output status when reading oplog

Currently the MongoDB connector silently processes the oplog and outputs no log messages with status or progress. This can be changed by moving to DEBUG logging, but that also introduces much more fine-grained output. The result is that it is difficult to understand if the connector is working.

Instead, the connector should output more information about its progress, perhaps periodically logging the number of records and the offset position (e.g., oplog position).