Summary:
After loading the persisted tablet server registry from the sys catalog, this diff adds logic to arrange for the ts count callback to be called if there are enough tablet servers in the registry.
Jira: DB-12649

Test Plan:
```
./yb_build.sh release --cxx-test-filter-re master_heartbeat-itest --cxx-test master_heartbeat-itest --gtest_filter GlobalTransactionTableCreationTest.CreateGlobalTransactionTableAfterFailover
```

Reviewers: asrivastava

Reviewed By: asrivastava

Subscribers: ybase, slingam

Differential Revision: https://phorge.dev.yugabyte.com/D40001