LU-1842 quota: add acq/rel logic on QMT

LU-1842 quota: add acq/rel logic on QMT

This patch adds the quota acquire/release logic on the QMT.
The QMT is in charge of distributing quota space across quota
slaves. It grants a significant amount of quota space (75%) initially
and then tries to balance spare quota space among slaves.
To do so, it sends glimpse on per-ID lock to notify slaves of qunit
shrink. qunit is now the maximal amount of available quota space a
slave can own. As for qtune, this value (typically 1/4 of qunit) is
used on the quota slave to decide when to release/pre-acquire quota
space.

Signed-off-by: Johann Lombardi <johann.lombardi@intel.com>
Change-Id: I8064ce614955421681ca1da7e2bb774001ca3bea
Reviewed-on: http://review.whamcloud.com/4178
Tested-by: Hudson
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>