Buffer Pool Manager (Project 1) Refactor

[connortsui20]

This PR will be the main PR for refactoring the buffer pool manager, which is the entirety of Project 1.

The overarching goal of this refactoring is to make the buffer pool manager API both thread-safe and type-safe. This means that use of raw pointers both internally and externally should be removed, and that thread-safe page guards should be the only way to interact with page data.

By doing this, we heavily limit the opportunities for misuse of the buffer pool manager API, which will in turn make the implementation and debugging of future projects (which will be much harder) easier.

Additionally, there will be implementation changes for the reference solution that will prefer indexing into arrays over raw pointers, limiting the opportunities for segfaults and memory corruption.

Here are the steps required for the refactor:

• Make any modifications to the LRU-K replacer and disk scheduler (change LRU-K Evict() signature #731)
• Change the BPM API in the public repo (d1fc14f)
  • Propagate the BPM API changes in the public repo (change occurrences of old methods to new ones)
  • Update tests that require OOM error / require old raw pointer API
• Update the writeup [in progress]
• Make all private repo changes (see private PR for more information [in progress])
• Update all test cases
• Update the starter code documentation
• [Potentially more steps necessary + review]

[connortsui20]

We need to also decide if we want to put the detailed implementation hints in the writeup (website) or keep them in the doc comments. Personally, I feel that documentation should always go right next to where the function implementation is, but I also understand that I made the implementation hints really long and detailed.