cthulhu: set appropriate min_size on pool create/update

Use an algorithm similar to Ceph's: requested min_size or
default, compare to size, use whichever is smaller; or if
no requested *or* default, use size - size/2.

On update, only recalculate min_size if size or min_size is given.

Fixes: #8820
Signed-off-by: Dan Mick <dan.mick@inktank.com>