Bug 509: Added In-memory datastore support for wildcarded change listeners

Bug 509: Added In-memory datastore support for wildcarded change listeners

Added support for wildcarded data change listeners,
which may register to updates of list entries (and their
subentries) without specifying key for list items.

Support for Wildcarded Data Change Listneners is done by
refactoring DataChangeEventResolver into Map/Reduce processing.

Map phase:
When visiting each modified node (with key) we also
looks for listener listening on same level without key.
This actually may trigger creation (mapping) of multiple events
for same listeners (eg. two flows we're added at the same time).

Reduce phase:
We walked thru all listeners to be notified and if they have
multiple events assigned, we merge them into one even to be
delivered.

Change-Id: Ic05ddae10bd0b316009cc5bafdeca76350f00390
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>