Add an option to trigger a CDS snapshot after import

A daexim import really means overwriting the contents of the datastore, which is a large operation, which ends up being stored in journal.

Doing multiple consecutive imports can cause the journal to grow, consuming both on-disk (leveldb file) space and in-memory (heap) space.

Add an option whereby after a successful import the CDS leader is instructed to take a snapshot, hence purging the now-stale journal entries – controlling consumption of both resources.