Turn on checkpointing for the keystore Data Store

Currently, the keystore data store uses a configuration of WiredTiger with logging enabled but without doing any checkpoints. This means as writes accumulate to the keystore, the log files (journal files) will continue to accumulate and never be deleted until shutdown time. WiredTiger has a built-in checkpointing thread that can be configured to run at a configured interval; we should either make use of this or run our own checkpoint thread (or use the current MongoDB checkpointing thread) to execute periodic checkpoints of the keystore.