Fix to not include a cookie in TCP mode.

b7098382 added a cookie always, however this only works in HTTP mode.
Remove/disable this automatic addition when we're in TCP mode.
Merge pull request #90 from bobtfish/fix_tcp_mode

Fix to not include a cookie in TCP mode.
Add 'use_previous_backends' option.

* This option defaults to true:  if there are no default servers and a watcher
  reports no backends, then use the previous backends that we already know
  about.
* Factor out code that updates '@backends' into the watcher base class.