mon: PaxosService: add helper function to check if a given version exists

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: convert osdmap_full as well

Store conversion wasn't converting the osdmap_full/ versions, only the
incrementals under osdmap/ and the latest full version stashed.  This
would lead to some serious problems during OSDMonitor's update_from_paxos
when the latest stashed didn't correspond to the first available
incremental.

Fixes: #4521

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge remote-tracking branch 'gh/wip-4521-fix' into next

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