feat(split): parent group update partition count

[hycdong]

Simple partition split process

1. meta receives client partition split request, and change partition count split: meta start partition split #286
2. replica notices partition count changed during on_config_sync feat(split): add splitting_replicas while on_config_sync #653
3. parent partition create child partition split: parent replica create child replica #291
4. parent prepare states for child to learn feat(split): parent replica prepare states #299
5. child partition async learn states from parent feat(split): child replica learn parent prepare list and checkpoint #309 feat(split): child replica apply private logs, in-memory mutations and catch up parent #319
6. child notify parent catch up feat(split): child notify parent catch up #390
7. update child group partition count feat(split): add update_child_group_partition_count #645
8. register child partition feat(split): register child partition #391
9. update parent group partition count

More partition split discussion in issue #69 and partition split design doc
This pr solves the part of ninth step of partition split, which is bold in process description.

What this pr solve

When primary parent registers child succeed, it will update its own partition_count, then broadcast this to secondary through group_check. Besides, this pull request also adds related unit test.