Some various small cleanups, optimizations, and fixes.

o Make fifo_batch=32 as default, from testing this appears a good
  default value. We still get good throughput, and latency is good.

o Reintroduce the merge_cleanup logic. We need it for deadline for
  rehashing requests when they have been merged.

o Cleanup last_merge logic. Move it to the new elv_merged_request(),
  this is where it really belongs. Doing it inside the io scheduler core
  can causes false positives, when the queue merge functions reject an
  otherwise good merge

o Have deadline_move_requests() account from last entry on the dispatch
  queue, if it is non-empty. It doesn't really matter what the last
  extracted sector was, if we are not right behind it.

o Clean/optimize deadline_move_requests()

o Account size of a request just a little bit. Streaming transfer isn't
  for free, it's just a lot cheaper than a seek.

o Make deadline_check_fifo() more readable.