Fixes a possible use-after-free in case of continue_ops() calling try_send(),
then connect_peer() -> set_timer() -> trigger_nearest() -> handle_op_part() -> continue_ops() again
OSDs often change their /pg/history keys during rebalance, so monitor receives additional
transaction failures from etcd if it re-runs lpsolve which sometimes may even lead to monitor
being unable to apply PG changes at all until rebalance completes
Also add protection from etcd watcher messages being split into multiple websocket
messages - I'm not sure if etcd actually does that, but it's better to have extra
protection anyway.
Also check that all etcd watchers are started in the keepalive routine, otherwise
it sometimes tries to revive etcd watchers starting with revision=1 which obviously
always fails because this revision is nearly always compacted.
All these changes should fix an old rarely reproduced bug where SOMETIMES OSDs
didn't react to PG config changes which was leading to offline pools on node reboot.
It happened on the full reload of state from etcd.
