Final TODOs for shared_queue class

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

New changes make mpmc perform slightly better in case of contention. Here's an
example with (note larger item values):

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

Boost Queue
      Enqueues (8x100000): 129ms
      Dequeues (8x100000): 75ms
      Fair mpmc (~50000000 items, 4 enq, 4 deq): 26982ms
      Enqueue-weighted mpmc (~50000000 items, 6 enq, 2 deq): 19083ms
      Dequeue-weighted mpmc (~50000000 items, 2 enq, 6 deq): 48055ms

Shared Queue
       Enqueues (8x100000): 128ms
       Dequeues (8x100000): 891ms
       Fair mpmc (~50000000 items, 4 enq, 4 deq): 7600ms
       Enqueue-weighted mpmc (~50000000 items, 6 enq, 2 deq): 8749ms
       Dequeue-weighted mpmc (~50000000 items, 2 enq, 6 deq): 5874ms