Work in progress

This work extends AIStateMachine to run multiplex() in the thread
that calls run(), cont() or set_state(). Note that all three
eventually call locked_cont(), so thats where multiplex() is called
from. Calling multiplex() means "running the state machine", as in
"calling multiplex_impl".

Currently only LLURLRequest uses this feature, and then only
for the HTTPGetResponder, and well only for the initializing,
start up and normal finish states.

A current/remaining problem is that we run into a situation where
the curl thread runs a statemachine to it's finish and kills it,
while the main thread is also 'running' it and tries to call
multiplex while the statemachine isn't running anymore.

indra/newview/lltexturefetch.cpp

@@ -338,6 +338,8 @@ public:
 	}
 #endif
 
+	/*virtual*/ bool thread_safe_complete(void) const { return true; }
+
 	/*virtual*/ void completedRaw(U32 status, const std::string& reason,
 							  const LLChannelDescriptors& channels,
 							  const buffer_ptr_t& buffer)