log: broadcast cond signals

We were using a single cond, and only signalling one waiter.  That means
that if the flusher and several logging threads are waiting, and we hit
a limit, we the logger could signal another logger instead of the flusher,
and we could deadlock.

Similarly, if the flusher empties the queue, it might signal only a single
logger, and that logger could re-signal the flusher, and the other logger
could wait forever.

Intead, break the single cond into two: one for loggers, and one for the
flusher.  Always signal the (one) flusher, and always broadcast to all
loggers.

Backport: bobtail, argonaut
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
(cherry picked from commit 813787af3dbb99e42f481af670c4bb0e254e4432)