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.
Backport of core#24656.
Depends on D16390.
There is no change in behavior, the RPCs are moved with no code change.