Summary:
Adds writes for the ts descriptor objects before committing on all code paths.

Flags added:
  - `persist_tserver_registry` - this is a test flag. It will be changed into an autoflag in a later diff. It toggles whether to write the tserver registry entries to the sys catalog when adding a new one. It also toggles whether to reload the tserver registry entries from disk when reloading the sys catalog.

Jira: DB-11843

Test Plan:
```
./yb_build.sh --cxx-test master_heartbeat-itest --gtest_filter 'MasterHeartbeatITest.TestRegistrationThroughRaftPersisted'
./yb_build.sh --cxx-test master-test --gtest_filter 'MasterTest.TestRegistrationThroughHeartbeatPersisted'
./yb_build.sh --cxx-test master-test --gtest_filter 'MasterTest.TestUnresponsiveMarkingPersisted'
./yb_build.sh --cxx-test master-test --gtest_filter 'MasterTest.TestHeartbeatFromRegisteredTSPersisted'
```

Reviewers: asrivastava, amitanand

Reviewed By: asrivastava

Subscribers: ybase, yql, slingam

Differential Revision: https://phorge.dev.yugabyte.com/D37279