osd: clean up osdmap sharing

osd: clean up osdmap sharing

- always use the Session::last_sent_epoch value, both for clients and osds
- get rid of the stl map<> of peer epochs
- consolidate all map sharing into a single maybe_share_map()
- optionally take a lower bound on the peer's epoch, for use when it is
  available (e.g., when we are handling a message that specifies what
  epoch the peer had when it sent the message)
- use const OSDMapRef& where possible
- drop osd->is_active() check, since we no longer have any dependency on
  OSD[Service] state beyond our osdmap

The old callchain was convoluted, partly because it was needlessly
separated into several layers of helpers, and partly because the tracking
for clients and peer OSDs was totally different.

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