osd: track clock delta between peer OSDs

osd: track clock delta between peer OSDs

We need to keep track of the monotonic clock deltas between peer OSDs in
order to be able to exchange timestamps across messages.  We need an
upper and lower bound on this delta, depending on the context where it is
used.

We can use the existing ping messages to get this by assuming that a ping
message is sent instantly to get a bound, and to share our delta in a
follow-up reply to share the other bound.  The ping sender will get both
bounds with a single ping + ping_reply exchange.  The ping receiver will
get the delta value from the next round's ping.

Include up_from in the ping messages to ensure we don't mix up different
instances of the same OSD.

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