resip/stack: TransactionState: change mWaitingForDnsResult to mPendingOperation enum type

git-svn-id: https://svn.resiprocate.org/rep/resiprocate/main@10703 ddefafc4-47db-0310-ae44-fa13212b10f2
resip/stack: TransportSelector: transmit now returns an enum, TransmitState

git-svn-id: https://svn.resiprocate.org/rep/resiprocate/main@10704 ddefafc4-47db-0310-ae44-fa13212b10f2
resip/stack: move post-send logic from sendCurrentToWire into helper method

git-svn-id: https://svn.resiprocate.org/rep/resiprocate/main@10705 ddefafc4-47db-0310-ae44-fa13212b10f2