Fix virtual channel proposal id generation

[manoranjith]

• Previously, the proposal ID for a virtual channel was generated using
  the ProposalID method defined on BaseChannelProposal.

• Hence, not all parameters of the virtual channel proposal were used.

• Fix it by defining a ProposalID method on the virtual channel.

• Also, remove the ProposalID method on the BaseChannelProposal, as it
  is not needed.

Fixes #299.

[matthiasgeihs]

I think we can achieve the same thing by using VirtualChannelProposal.Encode.

[matthiasgeihs]

We can reuse the VirtualChannelProposal.Encode method:

Suggested change

	// ProposalID returns the identifier of this channel proposal.
	func (p VirtualChannelProposal) ProposalID() (propID ProposalID) {
	hasher := newHasher()
	if err := perunio.Encode(hasher,
	p.Base(),
	p.Proposer,
	wire.Addresses(p.Peers),
	); err != nil {
	log.Panicf("proposal ID base encoding: %v", err)
	}
	for i := range p.Parents {
	if err := perunio.Encode(hasher, p.Parents[i]); err != nil {
	log.Panicf("proposal ID base encoding: %v", err)
	}
	}
	for i := range p.IndexMaps {
	for j := range p.IndexMaps {
	if err := perunio.Encode(hasher, (p.IndexMaps[i][j])); err != nil {
	log.Panicf("proposal ID base encoding: %v", err)
	}
	}
	}
	copy(propID[:], hasher.Sum(nil))
	return
	}
	// ProposalID returns the identifier of this channel proposal.
	func (p VirtualChannelProposal) ProposalID() (propID ProposalID) {
	hasher := newHasher()
	if err := perunio.Encode(hasher, p); err != nil {
	log.Panicf("encoding proposal: %v", err)
	}
	copy(propID[:], hasher.Sum(nil))
	return
	}

[manoranjith]

Yeah, I agree.

[manoranjith]

Updated in 248ecfc. Did the same for ledger channel and sub channel proposals, in the following commit: 2b3f79e.

[manoranjith]

As it was the only suggestion, I squashed the review commit and rebased the branch onto develop.