Reject received payments where PaymentRequest Expiry Tag time has been exceeded.

[n1bor]

Fixes #748

[sstone]

Nice! And choosing to return an unknown payment hash is consistent with our current behaviour. Just a few minor formatting changes/typos to fix and we're good. Thanks!

[pm47]

How about implementing this like so (loosely inspired by how guava's cleanes stale cache entries), that would result in a single path to UnknownPaymentHash:

1. Put the following code in a separate purgeExpiredRequests method:

hash2preimage.collect {
        case e@(_, (_, pr)) if pr.expiry.isEmpty => e // requests that don't expire are kept forever
        case e@(_, (_, pr)) if pr.timestamp + pr.expiry.get > currentSeconds => e // clean up expired requests
})

2. Create a PurgeExpiredRequests object to make the intent obvious:

case PurgeExpiredRequests =>
      context.become(run(purgeExpiredRequests(hash2preimage)))

3. In the handler for UpdateAddHtlc, use the newly created method:

purgeExpiredRequests(hash2preimage.get(htlc.paymentHash)) match {

[n1bor]

Not sure this is any better/clearer. I think I have done what you suggested.
I just wanted to avoid making it collect over all pending requests for every payment as that would scale very badly if getting 100's payments a minute/second...

[pm47]

I took the liberty to commit a new version that addresses your concerns. WDYT?

requested changes have been made, format is now correct