Catalog manager: don't lump together all updates from one tablet report

With current structure of the system tablet for rows storing metadata information on tablets, the catalog manager can create a very large write operation on the system tablet when processing full tablet reports sent from tablet servers. At some point (depends on the --rpc_max_message_size setting), a tablet report received from a tablet server comes through, but its Raft counterpart for the system tablet update doesn't because it might be almost two times larger. If that happens, Kudu cluster becomes almost non-functional because of self-perpetuating accepted-huge-tablet-report-but-cannot-push-Raft-update-to-follower-masters pattern.

The catalog manager should not lump together updates on all tablets received from one tablet server: https://github.com/apache/kudu/blob/3175c35c7d721aef0c4c6b358cc3b422089c1ba7/src/kudu/master/catalog_manager.cc#L4268-L4274