Clear old PG states, history, and OSD states on etcd state reload

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.

src/osd_cluster.cpp

@@ -649,7 +649,7 @@ void osd_t::apply_pg_config()
         {
             pg_num_t pg_num = kv.first;
             auto & pg_cfg = kv.second;
-            bool take = pg_cfg.exists && pg_cfg.primary == this->osd_num &&
+            bool take = pg_cfg.config_exists && pg_cfg.primary == this->osd_num &&
                 !pg_cfg.pause && (!pg_cfg.cur_primary || pg_cfg.cur_primary == this->osd_num);
             auto pg_it = this->pgs.find({ .pool_id = pool_id, .pg_num = pg_num });
             bool currently_taken = pg_it != this->pgs.end() && pg_it->second.state != PG_OFFLINE;