Refactor and optimize OMOP push processors

These changes pre-cache a mapping of natural keys to instance data
which drives the behavior of the processor.

Signed-off-by: Byron Ruth <b@devel.io>
Add convenience method on DataArchive to push staged data

Signed-off-by: Byron Ruth <b@devel.io>
Ensure connections are closed on database drop

Add a short max age for staging databases connections to prevent them
dangling around in case they are not closed properly.

Signed-off-by: Byron Ruth <b@devel.io>
Add stage command

Signed-off-by: Kevin Murphy <murphyke@email.chop.edu>