Add synchronous SDL readiness waiting API

Add synchronous SDL readiness waiting API

Add a new synchronous API function 'SyncStorage::waitReady()', which
can be used to wait SDL service readiness for the given namespace.
Function can be used to validate SDL service readiness for example
after application container start-up before any other
SDL operations (set, get, remove) are tried to do.
Also timeout value can be given to 'SyncStorage::waitReady()' function
to define expiration time for the case when readiness validation hangs
too long.

Issue-ID: RIC-226

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: Ic334cf04fa6c80f04a3c3e06b01fbf1ff7e940fd