rgw/sal: unify ReadStatsCB for async user/bucket stats

rgw/sal: unify ReadStatsCB for async user/bucket stats

`rgw::sal::Bucket::read_stats_async()` had returned stats as
`map<RGWObjCategory, RGWStorageStats>`, but the only caller in
rgw_quota.cc just summed up the categories into a single instance
of `RGWStorageStats`. moving this summation up into rgw_rados'
`RGWGetBucketStatsContext` allows us to unify these sal callbacks
around `RGWStorageStats` alone

the `RGWGetUserStats_CB`/`RGWGetBucketStats_CB` callbacks had inherited
from `RefCountedObject` and required manual reference counting. switch
to `boost::intrusive_ptr` for scope-based shared ownership

Signed-off-by: Casey Bodley <cbodley@redhat.com>