Inventory manager: get rid of synchronized blocks

This patch reworks the inventory manager to operate on batches, such
that it does not block notification threads and lowers the pressure on
the datastore.

Change-Id: I1953ce22446853b99a201381ff4d7b64a3cfcee7
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Inventory manager: get rid of synchronized blocks"