Added print_statemachine_diagnostics

The responder name is now cached in LLURLRequest
(ResponderBase::getName() must return a string literal).
The run time (in the main thread) per state machine is now accumulated
in AIStateMachine (instead of AIEngine::QueueElement).
When AIStateMachine::mainloop runs longer than StateMachineMaxTime
then a warning is printed that now includes the time spent in the
slowest state machine (that frame) and (if it is a LLURLRequest)
what the corresponding responder is. Also the total accumulated run
time of that state machine is printed.

From this is can be concluded that the only responder currently
regularly holding up the main thread is LLMeshLODResponder (mostly 30 to
100 ms, but with spikes in the 1 to 2 second range some times).