mgr: skip first non-zero incremental in PGMap::apply_incremental()

mgr: skip first non-zero incremental in PGMap::apply_incremental()

After initialization of PGMap instance PGMap::stamp is zero
and this cause huge first delta.
Also after mgr restart first non-zero value coming to PGMap::apply_incremental()
is current pg_sum value so it produces unreasonably huge pg_sum_delta.
This patch introduces a workaround to save pg_sum and not update pg_sum_delta
by first non-zero incremental.

Signed-off-by: Aleksei Gutikov <aleksey.gutikov@synesis.ru>
Fixes: http://tracker.ceph.com/issues/21773