Separate out {Identity,Equality}QueuedNotificationManager

Separate out {Identity,Equality}QueuedNotificationManager

There are use cases where we would like to use QNM on equality, i.e.
we do not realy have a listener instance, but rather a key to somewhere
else. That other place operates on equality, which makes it incompatible
with the current semantics.

This splits out the baseline queueing logic in from NotificationManager
interface, so that it can be reused to implement different execution
concepts.

QueuedNotificationManager is realized on top of it, so that the baseline
interface is implemented in AbstractQueuedNotificationManager and then
specialized to different lookups.

JIRA: YANGTOOLS-1038
Change-Id: I1123d21664a20ff380ccb7db7096801915491f6e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>