Keep track of the Init Features for every connected/channel'd peer

Keep track of the Init Features for every connected/channel'd peer

Since we want to keep track of the Init-context features for every
peer we have channels with, we have to keep them for as long as the
peer is connected (since we may open a channel with them at any
point).

We go ahead and take this opportunity to create a new per-peer-state
struct which has two levels of mutexes which is appropriate for
moving channel storage to.

Since we can't process messages from a given peer in parallel, the
inner lock is a regular mutex, but the outer lock is RW so that we
can process for different peers at the same time with an outer read
lock.