mon/Paxos: make 'paxos trim disabled max versions' much much larger

108000 is about 3 hours if paxos is going full-bore (1 proposal/second).
That ought to be pretty safe.  Otherwise, we start trimming to soon and a
slow sync will just have to restart when it finishes.

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