[WIP] Utilize temp cache and skip syncOffset during CATCHING_UP phase

[zx0401]

[changelog][BootstrappingVeniceChangelogConsumer] Utilize temp cache and skip syncOffset during CATCHING_UP phase to improve bootstrapping performance

Summary, imperative, start upper case, don't end with a period

• Skip syncOffset during CATCHING_UP phase
• During CATCHING_UP phase, we'll save raw data in a temporary hash map
• On completion of CATCHING_UP, we'll process records in temporary hash map and save in storage

How was this PR tested?

• unit test

Does this PR introduce any user-facing changes?

• [x] No. You can skip the rest of this section.
• [ ] Yes. Make sure to explain your proposed changes and call out the behavior change.