Final TODOs for shared_queue class

Updated README too, that class should be done for a while.

Attempted to have a short-circuit condition for dequeuers but that did not
measurably improve performance (the short-circuit condition checked if
head->next is null and if head is invalid, which would allow dequeuers
to have no tail loads on empty queues, but this didn't help).

Perf results:

vlad@vlad-W530:~/dev/Cpp-Util$ ./release/mpmc-test.exe bench boost
MPMC Queue Benchmark

Boost Queue
      Enqueues (8x100000): 122ms
      Dequeues (8x100000): 86ms
      Fair mpmc (~50000000 items, 4 enq, 4 deq): 26478ms
      Enqueue-weighted mpmc (~50000000 items, 6 enq, 2 deq): 18861ms
      Dequeue-weighted mpmc (~50000000 items, 2 enq, 6 deq): 46221ms

Shared Queue
       Enqueues (8x100000): 121ms
       Dequeues (8x100000): 882ms
       Fair mpmc (~50000000 items, 4 enq, 4 deq): 7432ms
       Enqueue-weighted mpmc (~50000000 items, 6 enq, 2 deq): 8288ms
       Dequeue-weighted mpmc (~50000000 items, 2 enq, 6 deq): 6632ms