Increase worker threads

1. As we delegate more things to background jobs we should increase the number of
   worker threads for sufficient systems.
2. 8GB systems are accepted as sufficient
3. Simplify test for backthumbs crawler and accept 8GB systems this way

src/common/darktable.c

@@ -1736,8 +1736,7 @@ int dt_init(int argc, char *argv[], const gboolean init_gui, const gboolean load
 
   darktable.backthumbs.running = FALSE;
   darktable.backthumbs.capable =
-      dt_get_num_procs() >= 4
-      && darktable.dtresources.total_memory / 1024lu / 1024lu >= 8000
+      dt_worker_threads() > 4
       && !(dbfilename_from_command && !strcmp(dbfilename_from_command, ":memory:"));
   if(init_gui)
   {
@@ -2089,9 +2088,7 @@ void dt_show_times_f(const dt_times_t *start,
 
 int dt_worker_threads()
 {
-  const int threads = dt_get_num_threads();
-  const int gbytes = (int)(_get_total_memory() / (1lu << 20));
-  const int wthreads = (gbytes >= 8 && threads >= 4) ? 6 : 3;
+  const int wthreads = (_get_total_memory() >> 19) >= 15 && dt_get_num_threads() >= 4 ? 7 : 4;
   dt_print(DT_DEBUG_DEV, "[dt_worker_threads] using %i worker threads\n", wthreads);
   return wthreads;
 }