Throttle: reduce lock hold periods

Throttle: reduce lock hold periods

Previously, we tended to dump a lot of log output under
the Throttle lock.  The log level for most log statements
has been reduced to 10.

Additionally, count and max are now atomic_t and can be
read without the Throttle lock.

Finally, most of the perf counter manipulations have been
moved outside of the lock.

Signed-off-by: Samuel Just <sam.just@inktank.com>