fix overtuning on flush thread

also removes more reasons for a large slab to be "dirty".

I was trying to avoid over-flushing when the global page pool is low,
but it looks like the thread doesn't really keep up and we end up not
moving pages fast enough. so the lower the global page pool the more
over-aggressive the flusher gets.

maybe instead of "oldest age" we could sort the large slabs by free
chunks?

slab_automove_extstore.c

@@ -175,13 +175,15 @@ void slab_automove_extstore_run(void *arg, int *src, int *dst) {
 
         // if page delta, oom, or evicted delta, mark window dirty
         // classes marked dirty cannot donate memory back to global pool.
-        if (a->iam_after[n].evicted - a->iam_before[n].evicted > 0 ||
-            a->iam_after[n].outofmemory - a->iam_before[n].outofmemory > 0) {
-            wd->evicted = 1;
-            wd->dirty = 1;
-        }
-        if (a->sam_after[n].total_pages - a->sam_before[n].total_pages > 0) {
-            wd->dirty = 1;
+        if (small_slab) {
+            if (a->iam_after[n].evicted - a->iam_before[n].evicted > 0 ||
+                a->iam_after[n].outofmemory - a->iam_before[n].outofmemory > 0) {
+                wd->evicted = 1;
+                wd->dirty = 1;
+            }
+            if (a->sam_after[n].total_pages - a->sam_before[n].total_pages > 0) {
+                wd->dirty = 1;
+            }
         }
 
         // reclaim excessively free memory to global after a full window

storage.c

@@ -618,17 +618,6 @@ static void *storage_write_thread(void *arg) {
         int target_pages = 0;
         if (global_pages < settings.ext_global_pool_min) {
             target_pages = settings.ext_global_pool_min - global_pages;
-            // make sure there's room in each class but don't over-flush.
-            // ie: we're going to want to move pages from the class with the
-            // "oldest" items, but this thread has no way of getting that info
-            // up-to-date. So we just free some pages in all active classes
-            // and let the page mover free stuff up.
-            // However, if the target delta is large (100+ megs) we can end up
-            // flushing gigabytes of data across many classes.
-            // Thus, we clamp the target here.
-            if (target_pages > 5) {
-                target_pages = 5;
-            }
         }
         counter++;
         if (to_sleep > settings.ext_max_sleep)