RFR: 8221102: Allow GC threads to participate in threads claiming protocol

[Kim Barrett]

Please review this change to the thread claiming protocol for parallel
iteration over threads. Previously, the protocol used a 1bit claim
counter plus zero as a "never claimed" marker. The new protocol uses a
uintx counter, while retaining zero as a "never claimed" marker.

JDK-8219613 changed use use of the claiming protocol, allowing all
threads to be included and claimed by a parallel iteration.  Doing so
introduced a bug.

Not all parallel iteration paths visit every thread. In particular,
possibly_parallel_threads_do (PPTD) only visits Java threads and the
VM thread. Using both all-thread parallel iterations and PPTD could
lead to some threads being improperly skipped, because of the 1bit
claim counter. For example, if an all-thread iteration is followed by
an even number of PPTD then another all-thread iteration, the second
all-thread iteration will incorrectly skip NonJavaThreads other than
the VMThread.

(Testing these changes with an instrumented claim function that
detected cases that would be in trouble with the old 1bit counter
found *many* occurrences.  It is somewhat surprising that this doesn't
seem to have produced lots of failures since JDK-8219613.)

We address this by expanding the claim counter to a uintx-sized
counter. There is still a theoretical possibility of collision, which
we deal with by resetting the claim value of all threads back to zero
when the global counter overflows. (On a 32bit platform, overflow of
the claim counter probably takes months. And even that is likely more
often than needed, since a collision needs MAX_UINTX-1 PPTD iterations
between all-thread iterations, which isn't how iterations are used.)

Some renaming was done to generalize from the old "parity"
nomenclature, and because it's not just about "oops_do" (and wasn't
before this change). Hopefully there won't be *too* much bike-shedding
on the names.

CR:
https://bugs.openjdk.java.net/browse/JDK-8221102

Webrev:
http://cr.openjdk.java.net/~kbarrett/8221102/open.00/

Testing:
mach5 tier1-5.