vinyl: get rid of vy_run_iterator->coio_read

Instead introduce vy_run_env_enable_coio(), which starts reader threads,
and make vy_run_iterator automatically switch to coio if reader threads
are running. With this patch, vy_read_iterator doesn't need a pointer to
vy_env to add a run as a source, only vy_run_env.

While we are at it, cleanup vy_conf a bit.

Needed for #1906