Refactored object cache

Refactored object cache

Now we do not perform object_cache_build_asyng till the end.
When we get new object_cache_build_async request we end ongoing
cache build and start new. This prevents some event flood case which
result in blocking event loop untill all events are processed.

Change-Id: Icb1c8b9c742709f94e0af756faba3036e73b8e44
Signed-off-by: Tomasz Olszak <t.olszak@samsung.com>