This commit introduces the chanacceptor package which is used
to determine, by a set of heuristics, which open channel messages
to accept and reject. Currently, two acceptors are implemented
via the ChannelAcceptor interface: ChainedAcceptor and RPCAcceptor.
The RPCAcceptor allows the RPC client to respond to the open channel
request, and the ChainedAcceptor allows a conjunction of acceptors
to be used.