htlcswitch/switch: clarify paymentID uniqueness

With the following commits, it'll become important to not resuse
paymentIDs, since there is no way to tell whether the HTLC in question
has already been forwarded and settled/failed.

We clarify this in the SendHTLC comments, and alter the tests to not
attempt to resend an HTLC with a duplicate payment ID.

htlcswitch/switch.go

@@ -348,7 +348,8 @@ func (s *Switch) ProcessContractResolution(msg contractcourt.ResolutionMsg) erro
 
 // SendHTLC is used by other subsystems which aren't belong to htlc switch
 // package in order to send the htlc update. The paymentID used MUST be unique
-// for this HTLC, otherwise the switch might reject it.
+// for this HTLC, and MUST be used only once, otherwise the switch might reject
+// it.
 func (s *Switch) SendHTLC(firstHop lnwire.ShortChannelID, paymentID uint64,
 	htlc *lnwire.UpdateAddHTLC,
 	deobfuscator ErrorDecrypter) ([sha256.Size]byte, error) {