Merge #7431: Rename permitrbf to mempoolreplacement and provide minimal string-list forward compatibility (needs 0.12 backport)

b922fbe Rename replacebyfee=opt-in to mempoolreplacement=fee (Luke Dashjr)
3b66e54 Simplify check for replacebyfee=opt-in (Luke Dashjr)
d65dee9 Accept replacebyfee=opt-in for turning on opt-in RBF (Luke Dashjr)
77b55a0 Rename permitrbf to replacebyfee (Luke Dashjr)