input_clock: extract clock_listener to a separate structure

Use a generic clock listener instead of the vlc_clock_t based-one so that we can attach a different listener to the input clock, in particular in the case of tests.

The change moves some code back to the es_out.c file, which we try to simplify, but the creation of the clock and buffering handling as a whole might be moved somewhere else someday.

This MR is also adding drift tracing to measure the current behaviour before changing it.

I'll use this MR to implement the beginning of a unified start mechanism afterwards.

Draft because it might be a good idea to start adding at least an automated smoke test.