Rename the term "atomic" in ReferenceProcessor

The ReferenceProcessor uses the term "atomic" to indicate "atomicity vs. the mutator" during reference discovery.

This somewhat unusual terminology causes lots of confusion for first time and often-time readers, mixing it up with atomic operations used elsewhere, and apparently causes long-standing bugs (e.g. JDK-8272985).

In practice it simply means whether the reference discovery is concurrent to the mutator or done in an STW pause.

Remove the use of "atomic" everywhere and replace it with something like "concurrent" or "in_stw" (probably preferably the former) as this much more commonly used and well understood.