Get as far as storing an incoming mail in crockery.db

DESIGN.md

@@ -38,3 +38,17 @@ following, though:
     * `Admin`
     * `Wildcard`
     * `PasswordHash`
+  * `Message`
+    * `ID`
+    * `Username`
+    * `Mailbox`
+    * `Header`
+    * `Body`
+
+I don't seem to be able to get accounts for a list of usernames very easily, or
+indexed-ly.
+
+The message body is being stored inefficiently (json []byte, so base64-encoded)
+
+Message username and mailbox are indexed, but is that good enough? Perhaps we
+should have a bucket per mailbox or something?