Add functionality to lookup an object and lock it. Additional the case is
handled that a object may be already in-destruction and the lookup will deny
returning the object.
The object_pool generalize the lookup and lock functionality of the rpc_server
and serve as base for following up patches to fix dangling pointer issues.
You cannot check an unsigned size_t variable for underflow, so I
changed the code to first check if an underflow would occur before
performing the subtraction.
Fixes#489.
