[C++] Reduce work done in locked section registerSubscription().
[C++] Reduce work done in locked section registerSubscription().

Change-Id: Ib734e3fbf660a7286fabe5e86c5e8cff0ccc44d5

clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,clive-jevons/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr