simplify further vsync handling

- we now clean-up "dead" connection in the main loop,
this entirely avoid the problem with the side effects of
releasing strong references. We now only hold on to strong
reference for the connection we will signal.

- also simplify how we build the list of "ready" connections, by
only adding them to the list when we did receive a vsync event

Change-Id: I2a84da431320a2af8e8a93e07622a1d258236f43