Summary:
Before this change, the LB would only prioritize under-replicated tablets for a given table. But
since the LB operates per table, it is possible that we end up performing a regular tablet move from
an earlier table over a under-replicated tablet add for a later table.

Fix: Fetching the list of under-replicated tablets during AnalyzeTablets then processing these tablets
first (before doing any regular load balancing). Also sorting this list of under-replicated tablets
by their number of replicas - this way we prioritize more under-replicated tablets first.
Jira: DB-9220

Test Plan:
```
ybd --cxx-test master_load_balancer_mocked-test --gtest_filter LoadBalancerRF5MockedTest.TestUnderReplicatedPriority
```

Reviewers: asrivastava, zdrudi

Reviewed By: zdrudi

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D39120