ObjectBox optimizations

[greenrobot-team]

Thanks for the work on this benchmark! I noticed the rather poor performance of ObjectBox on the tested iOS and Android devices and wanted to point out some things:

Test 4 + 5 call findAsync in a loop. Yet for other tests (including isar tests), operations are wrapped in a transaction. I suspect this could explain the poor performance on iOS + Android as the overhead of spawning an isolate is probably higher.

Test 16: similar thing here, multiple removeAllAsync calls are run (each one spawning an isolate) instead of using an async transaction like for the other tests (and isar).

I guess the take away for ObjectBox is that we need to improve our API docs on when to use an async transaction instead of calling the async variants of box operations multiple times (or even in a loop).