SERVER-13989 convert some callers to new quota API
SERVER-13951 Plumb OperationContext into DBDirectClients used for writing
SERVER-14395 Clean up StorageEngine initialization and shutdown

* Added StorageEngine::cleanShutdown() and commented that the destructor will
  never be called.

* MMapV1StorageEngine specific operations were pulled into its constructor
  and cleanShutdown implementation.

* StorageEngines are now constructed at a point where it is safe to spawn
  threads.