Revert "Free all slabs on region reset"

This reverts commit 67d7ab4.

The goal of the reverted commit was to fix flaky fails of tarantool
tests that checks amount of memory used by a fiber:

 | fiber.info()[fiber.self().id()].memory.used

It also attempts to overcome the situation when a fiber holds some
amount of memory, which is not used in any way. The high limit of such
memory is controlled by a threshold in fiber_gc() tarantool's function
(128 KiB at the moment):

 | void
 | fiber_gc(void)
 | {
 |         if (region_used(&fiber()->gc) < 128 * 1024) {
 |                 region_reset(&fiber()->gc);
 |                 return;
 |         }
 |
 |         region_free(&fiber()->gc);
 | }

The reverted commit, however, leads to significant performance
degradation on certain workloads (see #4736). So the revertion fixes the
performance degradation and opens the problem with tests, which is
tracked in #4750.

Related to #12
Related to tarantool/tarantool#4750
Fixes tarantool/tarantool#4736

small/region.h

@@ -156,16 +156,6 @@ region_reserve(struct region *region, size_t size)
 						       slab.next_in_list);
 		if (size <= rslab_unused(slab))
 			return (char *) rslab_data(slab) + slab->used;
-		/* Try to get a slab from the region cache. */
-		slab = rlist_last_entry(&region->slabs.slabs,
-					struct rslab,
-					slab.next_in_list);
-		if (slab->used == 0 && size <= rslab_unused(slab)) {
-			/* Move this slab to the head. */
-			slab_list_del(&region->slabs, &slab->slab, next_in_list);
-			slab_list_add(&region->slabs, &slab->slab, next_in_list);
-			return (char *) rslab_data(slab);
-		}
 	}
 	return region_reserve_slow(region, size);
 }
@@ -222,14 +212,14 @@ region_aligned_alloc(struct region *region, size_t size, size_t alignment)
 
 /**
  * Mark region as empty, but keep the blocks.
- * Do not change the first slab and use previous slabs as a cache to
- * use for future allocations.
  */
 static inline void
 region_reset(struct region *region)
 {
-	struct rslab *slab;
-	rlist_foreach_entry(slab, &region->slabs.slabs, slab.next_in_list) {
+	if (! rlist_empty(&region->slabs.slabs)) {
+		struct rslab *slab = rlist_first_entry(&region->slabs.slabs,
+						       struct rslab,
+						       slab.next_in_list);
 		region->slabs.stats.used -= slab->used;
 		slab->used = 0;
 	}