Check signatures before respend relay

Check that all inputs are completely valid before actually
relaying a double-spend.
Remove signal DoubleSpendDetected, use function

Also removes the need for forward reference to RelayableRespend.
Merge pull request #4450

0da6b3f Remove signal DoubleSpendDetected, use function (Tom Harding)
88dd359 Check signatures before respend relay (Tom Harding)