Update knowlege in both directions

[renepickhardt]

if we learn something from a channel in direction A --> B we currently update the A-->B arc in the uncertainty network. However we also learn something about the B-->A arc which we completely neglect as of now. This should be fixed before the release

[Lagrang3]

BTW. we have this in renepay plugin, here flow.h. We first realized that a bound in the liquidity [a,b), meaning that the liquidity satisfies a<=l0<b, implies that the liquidity in the opposite direction l1 is bounded by [a',b'), where c is the capacity of the channel, a'=c-b and b'=c-a.

When knowledge is updated in one direction, we update the [a,b) and we recompute [a',b').