Merge #13988: Add checks for settxfee reasonableness

Summary:
317f2cb3f4499afbaa63e3cac80567744f12c95b test: Check RPC settxfee errors (João Barbosa)
48618daf262b84c2e2f7322b5ca14375d7d68b64 Add checks for settxfee reasonableness (Anthony Towns)

Pull request description:

When using the `settxfee` RPC, the value is silently ignored if it is less than either than minrelaytxfee or the wallet's mintxfee. This adds an error response if that's going to happen, but still allows "settxfee 0" to deliberately default to the minimum value.

Tree-SHA512: ce685584cf8d6b9ca2cc97196d494220e3892b6a804a458086e04b3a23df281da432ad0a3053106a064c90c541ddb6f6b96a27cf8376d45af1e44449baf88456

Backport of Core PR13988
https://github.com/bitcoin/bitcoin/pull/13988/

Note: we don't have a wallet_bumpfee.py test, so the second commit (https://github.com/bitcoin/bitcoin/pull/13988/commits/317f2cb3f4499afbaa63e3cac80567744f12c95b) was added to rpc_estimatefee.py because it seemed to be the most closely related test

Test Plan:

make check
test_runner.py rpc_estimatefee

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4894