Incorporate suggestions from Richard's review of the LLPlugin changes.

Use LLMutexLock (stack-based locker/unlocker) for the straightforward cases instead of explicit lock()/unlock().

There are still a couple of cases (one overlapping lock lifetime and two loops that unlock the mutex to call another function inside the loop) where I'm leaving explicit lock/unlock calls.

Rename LLPluginProcessParent::sPollThread to sReadThread, for consistency.

Made the LLPluginProcessParent destructor hold mIncomingQueueMutex while removing the instance from the global list -- this should prevent a possible race condition in LLPluginProcessParent::poll().

Removed a redundant check when calling LLPluginProcessParent::setUseReadThread().