This PR adds an additional column to our feature table in BOLT 09, enumerating any dependencies that a feature pair has. At the moment, there are 3 such pairs that have dependencies, and more will be added in the near term. The motivation is to offer a concise way to describe these relations, and also simplify how we specify behavior gated by features with dependencies.

Consider the basic_mpp feature, which (as of #712) depends on payment_secret and transitively depends on var_onion_optin. Instead of specifying (or coding) has(basic_mpp) && has(payment_secret) && has(var_onion_optin) everywhere we need to gate basic_mpp, this allows us to isolate the gate to only basic_mpp since the transitive requirements are assumed to be already validated..

An implementation of this validation can be found here: https://github.com/lightningnetwork/lnd/blob/master/feature/deps.go which we are currently using as we build out MPP to simplify our own logic.

The test vectors have also been updated to set the var_onion_optin feature when setting a payment secret, as @t-bast points out, the current ones are missing this bit to be fully aligned with the existing spec.