Previously, we avoided calling term(index) for leader commit term check by keeping the first entry index for the current leader term in memory, and comparing against that index. (see #137826)

Now, since we have implemented term cache (which keeps track of a suffix of raft log entryIDs, and allow us to get the term of an entry by index), lets use the term cache for that check to reduce the complexity of our code. (see #142239 )

Jira issue: CRDB-48779