osdc/Objecter: maintain crush_location multimap

Observe and parse the 'crush location' config option.

Signed-off-by: Sage Weil <sage@inktank.com>
osdc/Objecter: use crush location and distance for LOCALIZE_READS

Use the hierarchy in the CRUSH map to determine what the closest
replica is.

Signed-off-by: Sage Weil <sage@inktank.com>
librbd: localize or distribute parent (snap) reads

The parent is always a snapshot.  We may want to treat it differently
than other snaps by virtue of it (likely) being a more highly-shared
image.

By default, localize parent reads.

Signed-off-by: Sage Weil <sage@inktank.com>
librados: deprecate aio_operate() read variant that takes snapid

The argument was ignored.

Also, mark get_version() using the fancy deprecated annotation.

Signed-off-by: Sage Weil <sage@inktank.com>