BUG-9028: make NonPersistentDataProvider schedule invocation

BUG-9028: make NonPersistentDataProvider schedule invocation

We need to make NonPersistentDataProvider behave in a fashion
similar to what PersistentDataProvider does for asynchronous
persistence calls, which is schedule execution of the provided
procedure rather than direct execution (which is fair for synchronous
execution).

In order to make that work we introduce ExecuteInSelfActor, which
has an executeInSelf() method, which uses internal mechanics to
schedule the call at a later point.

Change-Id: I116708d98154c8244ea80b4a1a1aa615abc3075d
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>