src/mgr/ClusterState.cc: micro improve ingest_pgstats

src/mgr/ClusterState.cc: micro improve ingest_pgstats

As I was going through ClusterState::ingest_pgstats
I noticed some refractor + mini-optimization we can do
with iterators and how we are inserting in a map container.
Propose caching the end() iterator of map
and use insert_or_assign() when adding new or updating values.
insert_or_assign() is better than [] in that pg_stat_t contain
some vectors and maps, so for adding new pgs or new keys
insert_or_assign() will do a direct copy-construction, rather than
default construction and assign values which arguable has more
overhead.

Signed-off-by: Kamoltat Sirivadhna <ksirivad@redhat.com>