BUG-1120: introduce NotificationListenerMap

Introduces encapsulation of the set of listeners, such that
modifications and lookup are isolated fromt the rest of the
implementation -- thus clearly defining the entry points.

Also fixes a possible iteration failure if concurrent modification
happens while a interest listener is being added.

Change-Id: I96e0260d372425d261b352b229ae293e638c287c
Signed-off-by: Robert Varga <rovarga@cisco.com>