MPP routing improvements

[t-bast]

I found a few things than can be improved in the MPP retry behavior.
Those will be particularly helpful for Trampoline-MPP to a remote peer (take into account the fact that a Trampoline node will likely have many channels).

Improvement 1: sending to a direct peer should not retry

It doesn't make any sense to let the PaymentLifecycle retry when we're forcing the choice of the first hop and the recipient is on the other end of that channel.
This is fixed by setting maxAttempts=1 when sending child payments via direct channels to the recipient.

Improvement 2: only use public channels for remote recipients

When the recipient isn't a direct peer, it's very unlikely that choosing a route prefix with an unannounced channel will succeed. Most unannounced channels Endurance/Horizon has are with mobile wallets. We ignore them when sending to remote nodes.

Improvement 3: handle RouteNotFound

We're choosing the first hop without looking at the graph. That means we may choose a first hop that can't yield a complete route to our recipient. When that happens, we should avoid retrying with that same hop.

If we have a lot of channels, it's likely that we'll often choose a bad first hop. Since we only retry 5 times, this will lead to many payment failures which is bad when we're relaying (Trampoline). So we add some randomness on whether to count a LocalFailure as a complete attempt or "refund" that attempt.

Note that this hack won't be necessary once we move the MPP logic inside the router: when that happens we will only select first hops that are on a potentially valid route to the recipient.

Unrelated

I also fixed the random failure that was happening in one of the MPP IntegrationSpec test.
With the code that's running on master, the PaymentInitiator doesn't guarantee that it will send the PaymentId before payment events.
Now we guarantee that.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@c76cc5b). Click here to learn what that means.
The diff coverage is 94.11%.

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Impacted Files	Coverage Δ
...r/acinq/eclair/payment/send/PaymentLifecycle.scala	90% <100%> (ø)
...r/acinq/eclair/payment/send/PaymentInitiator.scala	100% <100%> (ø)
...clair/payment/send/MultiPartPaymentLifecycle.scala	96.36% <92.3%> (ø)