*) Change memcache_ctxt to be not only a memory context structure, but
   also a thread local context structure.

*) To facilitate with the above goal, create an
   mcm_pthread_copy_ctxt() function that copies the function pointers
   and duplicates the various allocations in the ctxt.

*) Push all allocations of struct iovec into struct memcache_ctxt.

*) In fact, push all allocations of anything, buffers, etc, into
   struct memcache_ctxt.

*) Once the above has been completed, memcache(3) will be completely
   thread safe provided each thread has its own ctxt.

   Ex:
   	t = pthread_new();
	ctxt = mcm_pthread_copy_ctxt(existing_ctxt);
	t.execute_func(ctxt);
