rafthttp: refactor peer and add general stream

rafthttp/coder.go

@@ -0,0 +1,13 @@
+package rafthttp
+
+import "github.com/coreos/etcd/raft/raftpb"
+
+type encoder interface {
+	// encode encodes the given message to an output stream.
+	encode(m raftpb.Message) error
+}
+
+type decoder interface {
+	// decode decodes the message from an input stream.
+	decode() (raftpb.Message, error)
+}