bug: overlapping spark sends of one invoice both go out #1895

Description

Two overlapping Spark sends of one invoice can both go out. LightningRepo.admitSend admits a send whenever the key is free or held by the same rail, so a second Spark send of an invoice that is already in flight on Spark is admitted. Spark doesn't refuse a repeat.

This was already the case on master. #1893 widens the window, because a pre-restore invoice now waits for the operators' answer before the send.

To do

Refuse a second Spark send of a hash that already has a Spark send admitted or recorded. LDK refuses its own repeats, so the node rail can keep counting as it does now. Add a test that overlaps two Spark sends of one invoice and expects exactly one to reach the rail.

Twin: synonymdev/bitkit-ios (same gap in LightningService.admitSend). Found from greptile's review in synonymdev/bitkit-ios#1312 (comment).