Watch handling is unsound

[diwic]

The good news: Removing and adding watches is very rare and happens only during special conditions, e g while keeping Messages with a total length of > 63 MB in memory.

The bad news: This is not easily fixable, probably requires a rethink and a 0.6 release to get right.

The problems are (as more thoroughly explained by @albel727 in #72 ):

• When removing/disabling a watch, we need to process it immediately because DBus might close the fd immediately after the callback ends
• When adding/enabling a watch, we get a callback, but it can come from a separate thread, because calling dbus_message_unref can trigger it (and we have a impl Send for Message).