mon: restrict pool size to 1..10

See: #4159
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 5a457469839d0d1e6fc1aff1c8aeda6c8fd584dc)
osd: an interval can't go readwrite if its acting is empty

Let's not forget that min_size can be zero.

Fixes: #4159
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 5608d007dedf66bbdcf9998469f0edec350bc3a5)
Merge branch 'wip-4699' into bobtail