Merge #17564: rpc: Use mempool from node context instead of global

Summary:
fa8e650b525e9493bdfa393c0c3e34cb22c78c08 rest: Use mempool from node context instead of global (MarcoFalke)
fa660d65d7cc401ad5bbfdc076a074de19a79329 node: Use mempool from node context instead of global (MarcoFalke)
facbaf092f1ab298943206603cff6e6e3d30d452 rpc: Use mempool from node context instead of global (MarcoFalke)

Pull request description:

Currently they are identical, but in the future we might want to turn
the mempool into a unique_ptr. Replacing the global with the mempool
pointer from the node context simplifies this step.

ACKs for top commit:

jnewbery:
  Code review ACK fa8e650b5
ryanofsky:
  Code review ACK fa8e650b525e9493bdfa393c0c3e34cb22c78c08, Only the discussed REST server changes since the last review.

Backport of Core PR17564

Depends on D7965

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7966