Finally fixing the SlaveActivity message processing

In #35, @goldhoorn reported an issue with components that run in a SlaveActivity and proposed a patch to solve it. Calls to OwnThread operations of such a component have blocked until the next `execute()` call, which probably never happened if the master was not running. #60 partially reverted this patch and provided a different solution because the original patch broke use cases of SlaveActivities that do not have a master (reported in #59). Unfortunately, #60 was ineffective and only restored the original state.

This pull request hopefully fixes this issue following the comments on commit 2a012d66432bf2ec4983ece6118fc55d6376bf62 and adds a new unit test to test operation calls to slave components with and without callbacks.