mon: Paxos: bootstrap if we have fallen behind when we start collecting

Fixes: #4256

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
mon: Monitor: adjust sync start criteria considering a peon's bootstrap

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: check for an argument to 'quorum' before reading the array

Fixes: #4218

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: give useful errors to user when store is missing

We used to assert() instead, which didn't shed enough light on the cause
and could confuse the user into believing something *terrible* had
happened.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
PendingReleaseNotes: mention ceph-mon requiring the creation of data dir

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge remote-tracking branch 'gh/wip-4256'

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