BIP0144+wire: add a MessageEncoding variant for serialization/deserialization

This commit modifies the existing wire.Message interface to introduce a
new MessageEncoding variant which dictates the exact encoding to be
used when serializing and deserializing messages. Such an option is now
necessary due to the segwit soft-fork package, as btcd will need to be
able to optionally encode transactions/blocks without witness data to
un-upgraded peers.

Two new functions have been introduced: ReadMessageWithEncodingN and
WriteMessageWithEncodingN which wrap BtcDecode/BtcEncode with the
desired encoding format.

wire/blockheader.go

@@ -60,7 +60,7 @@ func (h *BlockHeader) BlockHash() chainhash.Hash {
 // This is part of the Message interface implementation.
 // See Deserialize for decoding block headers stored to disk, such as in a
 // database, as opposed to decoding block headers from the wire.
-func (h *BlockHeader) BtcDecode(r io.Reader, pver uint32) error {
+func (h *BlockHeader) BtcDecode(r io.Reader, pver uint32, enc MessageEncoding) error {
 	return readBlockHeader(r, pver, h)
 }
 
@@ -68,7 +68,7 @@ func (h *BlockHeader) BtcDecode(r io.Reader, pver uint32) error {
 // This is part of the Message interface implementation.
 // See Serialize for encoding block headers to be stored to disk, such as in a
 // database, as opposed to encoding block headers for the wire.
-func (h *BlockHeader) BtcEncode(w io.Writer, pver uint32) error {
+func (h *BlockHeader) BtcEncode(w io.Writer, pver uint32, enc MessageEncoding) error {
 	return writeBlockHeader(w, pver, h)
 }