trampoline: Add new channel allocation logic

The way we allocated channels for a trampoline payment could lead to a
case where we could get stuck selecting channels for a payment when the
rest amount was lower than the lower bound of the channel.

This commit introduces a new selection logic that tries to be greedy
first but will split payments more carefully if it is actually needded.

Signed-off-by: Peter Neuroth <pet.v.ne@gmail.com>

Author: nepet