Don't give up when missing the initial transmit window

[jonas-schievink]

Currently the link layer will abort the connection when it does not receive the initial connect event:

rubble/rubble/src/link/connection.rs

Lines 338 to 345 in 0c219f5

	// Master did not transmit the first packet during this transmit window.
	// TODO: Move the transmit window forward by the `connInterval`.
	// (do we also need to hop channels here?)
	self.conn_event_count += Wrapping(1);
	trace!("missed transmit window");
	Err(())

This shouldn't happen. Instead, we should do what the comment suggests (and check it against the spec).

The effect of this bug is that establishing a connection can spuriously fail, so this is a reliability issue.