Simplify incremental processing model

Another related question: could we make type of `pendingCommits` consistent with type of `commitsToProcess`, i.e. use `COMMIT` instead of `Instant`? Would it break anything for Hudi?
COMMITS uniquely identify a completed or inflight transaction and if needed can provide start and end times of a commit. Instant, on the other hand is a proxy for identifying a COMMIT.

_Originally posted by @ashvina in https://github.com/onetable-io/onetable/pull/129#discussion_r1375024492_

Start by tracing the incremental-processing code that defines pendingCommits and commitsToProcess, then review how Hudi represents COMMIT and Instant. Done means determining whether pendingCommits can use COMMIT consistently without breaking incremental processing or Hudi behavior, with the decision reflected in the relevant tests.

Written by the indexing model from the issue text.