queue: Implement persistency

This patch makes the queue read and write items to disk.

It uses protobuf for serialization. We serialize to text format to make
manual troubleshooting easier, as the performance difference is not very
relevant for us.

internal/queue/queue.proto

@@ -0,0 +1,38 @@
+
+syntax = "proto3";
+
+package queue;
+
+import "github.com/golang/protobuf/ptypes/timestamp/timestamp.proto";
+
+
+message Message {
+	// Message ID. Uniquely identifies this message, it is used for
+	// auditing and troubleshooting.
+	string ID = 1;
+
+	// The envelope for this message.
+	string from = 2;
+	repeated Recipient rcpt = 3;
+	bytes data = 4;
+
+	// Creation timestamp.
+	google.protobuf.Timestamp created_at_ts = 5;
+}
+
+message Recipient {
+	string address = 1;
+
+	enum Type {
+		EMAIL = 0;
+	}
+	Type type = 2;
+
+	enum Status {
+		PENDING = 0;
+		SENT = 1;
+		FAILED = 2;
+	}
+	Status status = 3;
+}
+