feat(split): add update_child_group_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
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
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 seventh step of partition split, which is bold in process description.

What this pr solve

When all children have caught up its parent's state, parent will send rpc to child to let them update partition count, child will update its partition count in memory and update .app_info file on disk. If all children update partition_count succeed, it will send register child rpc to meta server. Besides, this pr also adds related unit tests.

[acelyc111]

Seems there are some similar code on error handling paths, could you do some refactor?

[hycdong]

Yes, split error handling has little difference in different error cases, I will refactor it after I merge split core implementation.