Export double spends

[dagurval]

We need a method for exporting double spends detected.

This can be either:

• RPC call that serves a list of the last x number of [first seen, second seen] of double spending transactions.
• A new easily parsed log file + python parser example
• Something else.

This should be done by implementing the RespendAction interface and adding it to the action list in CreateDefaultAction. RespendLogger is a good example for how to implement an action.

doublespend.cash uses a hack to log double spends to a file (see README) and this should replace that.

This feature has also been requested here: dagurval/doublespend.cash#3