[Performance]: Profile & optimize the BlockManagerV2

[cadedaniel]

Proposal to improve performance

We've recently rewritten the block management subsystem for better testability. We need to profile it under real load to make sure it is performant enough to replace the block manager V1, and fix any issues.

We should do this once the block manager v2 is feature complete (still missing a few items).

Known issue:

• Prefix caching num_total_tokens is O(N^2) instead of O(N) (see https://github.com/vllm-project/vllm/pull/4142#discussion_r1585245813)