osd: Read balancer for OSDs with different sizes

osd: Read balancer for OSDs with different sizes

This commit adds calculation for desired primary distribution which
takes into account the osd size. This way smaller OSDs can take more
read operations (by adding more primaries) and the larger OSDs take less
primaries and the load of the cluater can increase. (This feature offset
a bit the weakest link in the chain effect under some conditions). In
order to calculate the loads correctly there is a need to know the
read/write ratio for the pool, and this commit assumes the read_ratio
parameter is available for the pool.

Signed-off-by: Josh Salomon <41079547+JoshSalomon@users.noreply.github.com>