osd: rework peer map epoch caching

osd: rework peer map epoch caching

We try to keep track of which epochs our peers have so that we can be
semi-intelligent about which map incrementals we send preceeding any
messages.  Since this is useful from the heartbeat and cluster channels/
threads, protect the data with an inner lock and clean up the callers.

Be smarter about when we forget.

Make note of peer epoch when we receive a ping.

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