- After a message is sent over the network (even UDS), it takes a
  non-trivial amount of time for a client to parse the message, reply
  and encode a new one. Therefore reading immediately from the network
  just wastes system calls. It has been observed that 'yield'-ing at
  this point saves these system calls, yielding to overall better
  performance.
  See http://www.yesodweb.com/blog/2014/02/new-warp


- Similarly, when an asynchronous writer finishes its job, it makes
  sense to give priority to other threads. This allows the other tasks
  to proceed, resulting in bigger batches of work for the asynchronous
  writer under higher loads.

Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>