Improve performance for certain queries/functions in REThreaded.

Some queries into RE were already thread safe, but now they are
documented as such and avoid jumping to another thread.

Additionally all void method in REThreaded are executed in an async
strategy.

Test: librenderengine_test
Bug: 184243497
Change-Id: Id04bcccf2c90acc3e6cb6f7716f9629639b757b7