Reset reconnect delay on manual reconnect, add delay event

This is just a hotfix workaround for an underlying problem. The _connect_routine code is "blocking" (in an async way) for connect_loop_wait seconds, so that a fresh-started manual reconnect will be silenty delayed. This code does the following changes:

1. It moves the delay to before the DNS resolution (with the exponential back-off it might well be that the DNS records are changed while slixmpp is waiting).

2. It adds a new event reconnect_delay that gets passed the number of seconds it will delay before actually reconnecting

3. It resets the connect_loop_wait timer on a manual connect/reconnect call to fix the interactive experience.

A proper fix would replace the sleep in _connect_routine with a properly timered re-invocation of it, but I don't understand enough of asyncio for pulling off that magic, and this is actually a proper improvement. Also I tested this and it works!