Add support for access token based authentication

* Extend the configuration format in order to allow access_token,
   user_id and device_id instead of username and password
 * Move the matrix login logic outside of login_and_sync for clarity
 * Add support for access token based session resuming instead of
   logging in every time (thus creating a new device each time the
   service starts up)
 * Delay the startup of feed reader loops until after the matrix module
   has had a chance to actually check authentication

This change is quite involved and there are a few caveats, namely an
intentional race condition between the feed reader loops and matrix
authentication, as well as significantly different behaviors depending
on which authentication scheme is being used: password based
authentication requires an API call while resuming a session using an
access token does not.

src/config.rs

@@ -27,12 +27,26 @@ pub struct FeedConfig {
     pub delay: u64
 }
 
+#[derive(Deserialize, Debug)]
+#[serde(untagged)]
+pub enum AuthConfig {
+    PasswordAuthConfig {
+        username: String,
+        password: String,
+    },
+    TokenAuthConfig {
+        user_id: String,
+        device_id: String,
+        access_token: String,
+    }
+}
+
 #[derive(Deserialize, Debug)]
 pub struct Config {
     pub default_room: String,
     pub feeds: Vec<FeedConfig>,
-    pub username: String,
-    pub password: String,
+    #[serde(flatten)]
+    pub auth: AuthConfig,
     pub homeserver_uri: String
 }