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):

MPMC Queue Benchmark

Boost Queue
      Enqueues (8x100000): 121ms
      Dequeues (8x100000): 82ms
      Fair mpmc (~50000000 items, 4 enq, 4 deq): 31316ms
      Enqueue-weighted mpmc (~50000000 items, 6 enq, 2 deq): 19307ms
      Dequeue-weighted mpmc (~50000000 items, 2 enq, 6 deq): 21443ms

Shared Queue
       Enqueues (8x100000): 107ms
       Dequeues (8x100000): 914ms
       Fair mpmc (~50000000 items, 4 enq, 4 deq): 7447ms
       Enqueue-weighted mpmc (~50000000 items, 6 enq, 2 deq): 8362ms
       Dequeue-weighted mpmc (~50000000 items, 2 enq, 6 deq): 6420ms