Multi-threaded implementation of RMA for distributed shared memory

MPID_Win_fence(assert,win)

* Mark the window epoch as closed so that we can

  detect attempts to perform unsynchronized RMA operations.

  NOTE: In the case of multiple threads calling RMA operations,

  closing the window epochs without waiting for other threads to

  finish their RMA calls will likely result in a race condition if the

  application is not properly synchronized.  This is really an

  application error, so in the interest of high-performance, we make

  no attempt to minimize the non-determinism.

* Atomically move the remote handler call counters to a buffer and

  zero the counters.  This ensures that the 

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