Move retry-limiting to `retry_payment_with_route`

Move retry-limiting to `retry_payment_with_route`

The documentation for `Retry` is very clear that it counts the
number of failed paths, not discrete retries. When we added
retries internally in `ChannelManager`, we switched to counting
the number if discrete retries, even if multiple paths failed and
were replace with multiple MPP HTLCs.

Because we are now rewriting retries, we take this opportunity to
reduce the places where retries are analyzed, specifically a good
chunk of code is removed from `pay_internal`.

Because we now retry multiple failed paths with one single retry,
we keep the new behavior, updating the docs on `Retry` to describe
the new behavior.