SYNCIDB: IPC thread refactoring and cleanup.

--HG--
rename : dom/workers/ipc/PWorkerModule.ipdl => dom/workers/ipc/PWorkerPool.ipdl
rename : dom/workers/ipc/WorkerModuleChild.cpp => dom/workers/ipc/WorkerPoolChild.cpp
rename : dom/workers/ipc/WorkerModuleChild.h => dom/workers/ipc/WorkerPoolChild.h
rename : dom/workers/ipc/WorkerModuleParent.cpp => dom/workers/ipc/WorkerPoolParent.cpp
rename : dom/workers/ipc/WorkerModuleParent.h => dom/workers/ipc/WorkerPoolParent.h