Fixes a race condition when cancelling requests on connection lost

[anarthal]

• Changes how cancel_on_conn_lost is used to ensure it is called only once and after the reader and writer tasks have exited.
• This fixes a problem in test_conn_reconnect
• Adds a test for multiplexer::reset()
• Adds stronger invariants to the multiplexer functions to be called by the reader and writer
• Removes test_issue_181, since the same functionality is being covered by unit tests already
• Removes basic_connection::run_is_canceled