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>
Datastore: Reuse successful future

ImmediateFuture does not keep any state, so we can reuse a single
instance, preventing garbage buildup just a tiny bit.

Change-Id: I200662a666f4529ea82a2ac3948be9536ca32281
Signed-off-by: Robert Varga <rovarga@cisco.com>
BUG-1520 coverage tests for config-manager

Change-Id: Iec63877296f4dcea2971dee38996b334099c57b9
Signed-off-by: Filip Tehlar <ftehlar@cisco.com>
Merge "Datastore: lower in-memory change tracking overhead"
Merge "Datastore: Reuse successful future"
Merge "BUG-1520 coverage tests for config-manager"