Move `Channel` fields into `ChannelContext` struct

Move `Channel` fields into `ChannelContext` struct

This is a first step for simplifying the channel state and introducing
new unfunded channel types that hold similar state before being promoted
to funded channels.

Essentially, we want the outer `Channel` type (and upcoming channel types)
to wrap the context so we can apply typestate patterns to the that wrapper
while also deduplicating code for common state and other internal fields.