watchtower: add channelID index

In this commit, a new channel-ID index is added to the tower client db
with the help of a migration. This index holds a mapping from a
db-assigned-ID (a uint64 encoded using BigSize encoding) to real channel
ID (32 bytes). This mapping will help us save space in future when
persisting references to channels.

watchtower/wtdb/migration3/codec.go

@@ -0,0 +1,19 @@
+package migration3
+
+import (
+	"encoding/hex"
+)
+
+// ChannelID is a series of 32-bytes that uniquely identifies all channels
+// within the network. The ChannelID is computed using the outpoint of the
+// funding transaction (the txid, and output index). Given a funding output the
+// ChannelID can be calculated by XOR'ing the big-endian serialization of the
+// txid and the big-endian serialization of the output index, truncated to
+// 2 bytes.
+type ChannelID [32]byte
+
+// String returns the string representation of the ChannelID. This is just the
+// hex string encoding of the ChannelID itself.
+func (c ChannelID) String() string {
+	return hex.EncodeToString(c[:])
+}