ZOOM: In-memory implementation of the Storage trait

The patch provides a simple, in-memory implementation of the Storage
trait to use with unit tests. The class receives a Reactor instance
as constructor argument, which specifies the thread on which the
observable notifications are executed.

Ref: MN-3065

Change-Id: I749ba0cbf4cb5de16b9a8b267eb895ec9ee02ca9
Signed-off-by: Alex Bikfalvi <alex.bikfalvi@midokura.com>
Merge "ZOOM: In-memory implementation of the Storage trait"