mon, osd: per pool space-full flag support

mon, osd: per pool space-full flag support

The newly introduced 'device-class' can be used to separate
different type of devices into different pools, e.g, hdd-pool
for backup data and all-flash-pool for DB applications.

However, if any osd of the cluster is currently running out
of space (exceeding the predefined 'full' threshold), Ceph
will mark the whole cluster as full and prevent writes to all pools,
which turns out to be very wrong.

This patch instead makes the space 'full' control at pool granularity,
which exactly leverages the pool quota logic but shall solve
the above problem.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>