Remove unnecessary complexity of isStarted/startFlow methods and implicit accompanying state.



git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/tubes-1956-2@36822 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb
Failing test case for backpressure via a Tube's internal buffer.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/tubes-1956-2@36823 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb
Make that test pass.  I guess it wasn't very good

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/tubes-1956-2@36824 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb