mon: fix synchronise pgmap with others

mon: fix synchronise pgmap with others

leader mon down for a long time and then the local pgmap/osdmap was behind latest map a lot,
when it start and sync ,it will read full map from other mons;
if serveral osds/pgs were deleted during mon offline, these osds/pgs will be left over in pgmap until manully restart ceph-mon process

Solution:
reset pgmap before read pgmap full,make sure local maps are the same as the latest map

Signed-off-by: z09440 <zhao.mingyue@h3c.com>
Signed-off-by: song baisen <song.baisen@zte.com.cn>