vinyl: cleanup and actualize a few configuration parameters

* remove dump_age_period; not much added value compared to
  dump_age itself;
* remove dump_wm; dump_wm is a global constant that
  sets the maximal run size in a range; it must be a function of
  range size, which is defined per space; it's better to estimate
  it automatically (we may vary run size depending on the
  temperature of the run).
* remove dump_prio; we're abandoning zone-based scheduler,
  and this was merely a handle to prioritize dumps of runs in red
  zone;
* provide reasonable defaults for dump_age (40 seconds) and
  compact_wm (begin merge when there are at least two runs in a range)
* update tests