[C++] Protect async arbitration callback
[C++] Protect async arbitration callback

* Async arbitration callback captures shared_ptr to ProxyBuilder
  so that ProxyBuilder cannot be destroyed while it runs.

Change-Id: I24a1029f6c6f0964bd0f3eab29c32c97ffd885ba

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