Fix (and test) threaded payment retries

Fix (and test) threaded payment retries

The new in-`ChannelManager` retries logic does retries as two
separate steps, under two separate locks - first it calculates
the amount that needs to be retried, then it actually sends it.
Because the first step doesn't udpate the amount, a second thread
may come along and calculate the same amount and end up retrying
duplicatively.

Because we generally shouldn't ever be processing retries at the
same time, the fix is trivial - simply take a lock at the top of
the retry loop and hold it until we're done.