Add zone vector cache.

Zone children will be now collected in staging vectors. When the zone is
ended (and no children can be added anymore to it), a size-fitted vector
is allocated using slab allocation. The over-allocated vector is then
put into cache for use in future zones.

This is only active for vectors <= 8192 elements, or 64 KB (chosen
arbitrarily), to reduce time spent on copying memory.

Overall, this change should have the following effects:
- System memory allocation pressure reduction, due to re-usage of
  vectors, which eliminates the need for constant growth.
- Reduction of memory usage, because children vectors are now fitted to
  required size.
- Slight increase of zone processing time, due to memory copying?

server/TracyWorker.hpp

@@ -178,6 +178,8 @@ private:
         Vector<Vector<ZoneEvent*>> zoneChildren;
         Vector<Vector<GpuEvent*>> gpuChildren;
 
+        Vector<Vector<ZoneEvent*>> zoneVectorCache;
+
         CrashEvent crashEvent;
     };