- Separates the trampoline and local routing multi-part payment cases.
- Ask only for splits that don't send over a single channel (those have
been tried already in the single-part case).
- Makes sure that create_routes_for_payment only yields partial routes
that belong to a single split configuration.
- Tracks trampoline fee levels on a per node basis, previously, in the
case of having two channels with a trampoline forwarder, the global
fee level would have increased by two levels upon first try.
- The splitting algorithm is redesigned to use random distribution of
subsplittings over channels.
- Splittings can include multiple subamounts within a channel.
- The single-channel splittings are implicitly activated once the
liquidity hints don't support payments of large size.
A penalty is added for split configurations which saturate a channel.
Saturation of channels is discouraged as we don't know the fees
beforehand. The penalty is accomplished via an exponential function that
kicks in when the subamount reaches about the total funds available
(this amount is controlled by the parameter EXHAUST_DECAY_FRACTION).
