mon: Paxos: bootstrap leader if he has fallen behind upon reaching collect

Fixes: #4256

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
mon: Paxos: increase trim tolerance from 3 to 30.

This increase only means that we'll keep more versions around before we
trim.  It doesn't change the number of versions we'll keep around after
trimming (that's still as much as 'paxos_max_join_drift', i.e. 10), nor
does it change the criteria used to consider a monitor as having drifted
(same rule applies, 'paxos_max_join_drift').

This change however will enable the leader to put off trimming for a longer
period of time, giving a better chance for a monitor to join the cluster.
See, after going through the probing phase, at which point a monitor may
only be, say, 5 versions off, the same monitor may end up getting into the
quorum only to find that in-between probing and finally triggering an
election some 6 versions might have come to existence.  Before this patch,
by then the state had been trimmed and the monitor would have to bootstrap
to perform a full store sync.  With this patch in place, the monitor would
be able to sync the remaining 11 versions.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>