Sierra provides clock_gettime(), but not pthread_condattr_setclock.
So we had better lot try to use CLOCK_MONOTONIC as our source for
time when waiting, since we ccan never actually tell the condition
that we mean CLOCK_MONOTONIC.

This isn't a tor bug yet, since we never actually pass a timeout to
tor_cond_wait() outside of the unit tests.

Not telling the cmux would sometimes cause an assertion failure in
relay.c when we tried to get an active circuit and found an "active"
circuit with no cells.

Additionally, replace that assert with a test and a log message.

Fix for bug 20203. This is actually probably a bugfix on
0.2.8.1-alpha, specifically my code in 8b4e5b7e where I
made circuit_mark_for_close_() do less in order to simplify our call
graph. Thanks to "cypherpunks" for help diagnosing.

Our use of the (mockable) tor_close_socket() in the util/socket_..
tests confused coverity, which could no longer tell that we were
actually closing the sockets.

 * FIXES #20201: https://bugs.torproject.org/20201