Retry PG changes without re-running lpsolve when pool configuration and OSD tree don't change

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

tests/test_change_pg_count.sh

@@ -18,6 +18,7 @@ try_change()
     for i in {1..6}; do
         echo --- Change PG count to $n --- >>testdata/osd$i.log
     done
+    echo --- Change PG count to $n --- >>testdata/mon.log
 
     $ETCDCTL put /vitastor/config/pools '{"1":{'$POOLCFG',"pg_size":'$PG_SIZE',"pg_minsize":'$PG_MINSIZE',"pg_count":'$n'}}'