When more than one invitation socket is opened, Alice should pick which one to use and Bob should use whichever one Alice picks. This fixes a race condition where each party picked a different socket and closed the other.
DuplexOutgoingSession flushes its output stream if it's idle for a
transport-defined interval, causing an empty frame to be sent. The TCP
and Tor plugins use a socket timeout equal to twice the idle interval to
detect dead connections.
See bugs #27, #46 and #60.
