TCP - Reduce context switching when handling large send*() or write()'s

TCP - Reduce context switching when handling large send*() or write()'s

* Large sends or writes would cause sosend() to sleep and wakeup for every
  tcp segment acknowledged, resulting in massive inefficiencies.  To deal
  with this, if no LOWAT is set we automatically set the LOWAT to 1/2 the
  send buffer size and adjust the wakeup code.