LU-3692 api: More flexible logging API.

LU-3692 api: More flexible logging API.

Allow applications to register their own logging callback instead of
writing every log message to stderr/stdout. Registered callbacks are
global to the process, just like the log level.

Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Change-Id: I5ccfd1e24f1786408ce67b2cd87d53443bcd028a
Reviewed-on: http://review.whamcloud.com/7572
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>