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.

test/t-03-queue_persistency/run.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+
+set -e
+. $(dirname ${0})/../util/lib.sh
+
+init
+
+# Add an item to the queue before starting chasquid.
+go run addtoqueue.go --queue_dir=.data/queue \
+		--from someone@testserver \
+		--rcpt someone@testserver \
+		< content
+
+generate_certs_for testserver
+
+mkdir -p .logs
+chasquid -v=2 --log_dir=.logs --config_dir=config &
+wait_until_ready 1025
+
+# Check that the item in the queue was delivered.
+wait_for_file .mail/someone@testserver
+
+mail_diff content .mail/someone@testserver
+
+success