mon: uninline _trim_enable and Paxos::trim_{enable,disable} so we can debug them

Signed-off-by: Sage Weil <sage@inktank.com>
mon/Paxos: fix sync restart

If we have a sync going, and an election intervenes, the client will
try to continue by sending a new start_chunks request.  In order to
ensure that we get all of the paxos commits from our original starting
point (and thus properly update the keys from which they started),
only pay attention if they *also* send their current last_committed
version.  Otherwise, start them at the beginning.

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