Allow ReferenceProcessor to always be MT processing

Currently, the mt_processing flag is adjusted for each phase depending on the number of workers: if it's 1 worker, then mt_processing is disabled, for more than 1 worker, mt_processing is enabled. Depending on the flag, ref-processing is either done through AbstractRefProcTaskExecutor, or directly in VMThread. However, we have a situation where we always want to execute via the executor, and never directly. Instead we decide in the executor whether or not we want to execute directly in the VMThread.