osd: add pg_pool_t::NO_OMAP flag

When an erasure coded pool is created set the NO_MAP pg_pool_t::flag.
Add the pg_pool_t::can_omap() accessor for tiering pools to check if
the other pool supports OMAP operations.

http://tracker.ceph.com/issues/6895 fixes #6895

Signed-off-by: Loic Dachary <loic@dachary.org>
osd: add osd pool set/get no_omap

so omap support can be set on a per pool basis (unless it is erasure
coded in which case it is always disabled).

Signed-off-by: Loic Dachary <loic@dachary.org>