This commit modifies most of the wire messages to uniquely identify any
*active* channels by their funding output. This allows the wire
protocol to support funding transactions which open several channels in
parallel.
Any pending channels created by partial completion of the funding
workflow are to be identified by a uint64 initialized by both sides as
follows: the initiator of the connection starts from 0, while the
listening node starts from (1 << 63). These pending channel identifiers
are expected to be monotonically increasing with each new funding
workflow between two nodes. This identifier is volatile w.r.t to each
connection initiation.
