common/AsyncReserver: add dump()

Signed-off-by: Sage Weil <sage@inktank.com>
osd: add dump_reservations asok command

Allow an admin to inspect pending and in-progress reservations.

Backport: firefly
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #2059 from ceph/wip-osd-dumpres

osd: allow dumping reservation state via asok

Reviewed-by: Samuel Just <sam.just@inktank.com>