refactor: Move mempool RPCs to rpc/mempool

[maflcko]

This moves the remaining mempool RPCs to rpc/mempool. Previously all mempool RPCs from the blockchain category have been moved. This patch moves the ones from the rawtransactions category.

In the future, as a follow-up to this refactoring patch, it could be considered whether a new mempool category should be introduced.

Beside a clearer code organization, this pull request should also reduce the compile time and space of the rawtransactions.cpp file.

[promag]

Code review ACK fac5a51.

[DrahtBot]

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #24661 (refactor: Use clang-tidy syntax for C++ named arguments by fanquake)
• #24513 (CChainState -> Chainstate by jamesob)
• #21422 (Add feerate histogram to getmempoolinfo by kiminuo)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[shaavan]

Concept ACK

• The static keyword is used appropriately in rpc/mempool.cpp file. I checked for any other instances where a static keyword could be used and found none.
• The moved code looks good to me. I was able to verify the move-only changes in the second commit successfully.
• I was able to compile the PR successfully on Ubuntu 20.04

[shaavan]

nit:

I would suggest putting these two lines after the rest of the blockchain category functions to match the proper lexicographical ordering.

[maflcko]

I am expecting that everything will move to a new mempool category later, so I am leaving as-is for now.

[shaavan]

I think that's acceptable reasoning.
Other than this, I can't find any blockers for this PR, and hence I think it can be merged.

ACK fac5a51