messages/MPGStats: do not set paxos version to osdmap epoch

messages/MPGStats: do not set paxos version to osdmap epoch

The PaxosServiceMessage version field is meant for client-coordinated
ordering of messages when switching between monitors (and is rarely
used).  Do not fill it with the osdmap epoch lest it be compared to a
pgmap version, which may cause the mon to (near) indefinitely put it on
a wait queue until the pgmap version catches up.

Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>