When the recipient is a direct peer, we can use the accurate knowledge
of our local channels instead of hard-coded MPP parameters to choose
the number of routes and the minimum route amount.

This change makes it possible to easily send all local funds in one
payment when we're directly connected to the recipient.

The routing hint we get in a Bolt 11 invoice may be obsolete when we attempt
the payment: one of the nodes in the route may have updated his relay fees.

Since this affects a private channel that is not kept in the routing graph,
we need to update the routing hints before injecting them in the router.
This was already done in `PaymentLifecycle` with automatic retries, but when
using MPP we retried in the `MultiPartPaymentLifecycle` instead of inside
the `PaymentLifecycle`, so we need to handle routing hints updates there.

* Use secp256k1-kmp

We get rid of our old JNI wrapper and switch to bitcoin-lib 0.18 which is based on secp256k1-kmp.
This will give us a consistent secp256k1 library on all our apps (since secp256k1-kmp also provides an Android library).
We also switch from spongycastle to bouncycastle (bitcoin-lib 0.18 uses bouncycastle).

* Set version to SNAPSHOT

Co-authored-by: dpad85 <5765435+dpad85@users.noreply.github.com>

Remove a legacy work-around for very old eclair-mobile users.