Fix a rare infinite-recursion bug when shutting down.

Fix a rare infinite-recursion bug when shutting down.

Once we had called log_free_all(), anything that tried to log a
message (like a failed tor_assert()) would fail like this:

   1. The logging call eventually invokes the _log() function.
   2. _log() calls tor_mutex_lock(log_mutex).
   3. tor_mutex_lock(m) calls tor_assert(m).
   4. Since we freed the log_mutex, tor_assert() fails, and tries to
      log its failure.
   5. GOTO 1.

Now we allocate the mutex statically, and never destroy it on
shutdown.

Bugfix on 0.2.0.16-alpha, which introduced the log mutex.

This bug was found by Matt Edman.