Datastore: lower in-memory change tracking overhead

We needlessly track the reference to notification manager inside the
even tasks. Instead simplify the return to a Map.Entry and pipe the
resulted data into the manager. This reduces the temporary size of a
tracker, lowering the rate at which we produce garbage.

Change-Id: Ib390d0e65ce1583371a2e887525d9b7ee0392396
Signed-off-by: Robert Varga <rovarga@cisco.com>