osd: implement read balancer

osd: implement read balancer

This commit implements two functions:

1. calc_desired_primary_distribution
   Based on the number of pgs in a pool and the pool's
   replica count, we calculate the ideal number of primary
   pgs that should be assigned to each OSD on that pool in
   order for reads to be balanced.

2. balance_primaries
   This is the overall algorithm used to balance reads (primary
   pgs) in a pool. Based on the first function, we re-distribute
   primaries on the OSDs in a pool so each OSD has the ideal
   number of primaries. This is done without data movement.

Signed-off-by: Laura Flores <lflores@redhat.com>