Enforce minRelayTxFee on wallet created tx and add a maxtxfee option.

Previously the minRelayTxFee was only enforced on user specified values.

It was possible for smartfee to produce a fee below minRelayTxFee which
 would just result in the transaction getting stuck because it can't be
 relayed.

This also introduces a maxtxfee option which sets an absolute maximum
 for any fee created by the wallet, with an intention of increasing
 user confidence that the automatic fees won't burn them. This was
 frequently a concern even before smartfees.

If the configured fee policy won't even allow the wallet to meet the relay
 fee the transaction creation may be aborted.
contrib: make github-merge require signing

- Exit when there is no signing key configured
- Exit when user does not want to sign off
Reject headers that build on an invalid parent
Merge pull request #5517

39c8099 contrib: make github-merge require signing (Wladimir J. van der Laan)
Merge pull request #5459

3497022 Reject headers that build on an invalid parent (Pieter Wuille)
Merge pull request #5485

aa279d6 Enforce minRelayTxFee on wallet created tx and add a maxtxfee option. (Gregory Maxwell)