chan_console: Fix deadlock caused by unclean thread exit.

chan_console: Fix deadlock caused by unclean thread exit.

To terminate a console channel, stop_stream causes pthread_cancel
to make stream_monitor exit. However, commit 5b8fea93d106332bc0faa4b7fa8a6ea71e546cac
added locking to this function which results in deadlock due to
the stream_monitor thread being killed while it's holding the pvt lock.

To resolve this, a flag is now set and read to indicate abort, so
the use of pthread_cancel and pthread_kill can be avoided altogether.

Resolves: #308