RFR: 8200557: OopStorage parallel iteration scales poorly

[Kim Barrett]

Please review this change to OopStorage parallel iteration to improve
the scaling with additional threads.

Two sources of poor scaling were found: (1) contention when claiming
blocks, and (2) each worker thread ended up touching the majority of
the blocks, even those not processed by that thread.

To address this, we changed the representation of the sequence of all
blocks.  Rather than being a doubly-linked intrusive list linked
through the blocks, it is now an array of pointers to blocks.  We use
a combination of refcounts and an RCU-inspired mechanism to safely
manage the array storage when it needs to grow, avoiding the need to
lock access to the array while performing concurrent iteration.

The use of an array for the sequence of all blocks permits parallel
iteration to claim ranges of indices using Atomic::add, which can be
more efficient on some platforms than using cmpxchg loops.  It also
allows a worker thread to only touch exactly those blocks it is going
to process, rather than walking a list of blocks.  The only
complicating factor is that we have to account for possible overshoot
in a claim attempt.

Blocks know their position in the array, to facilitate empty block
deletion (an empty block might be anywhere in the active array, and we
don't want to have to search for it).  This also helps with
allocation_status, eliminating the verification search that was needed
with the list representation.  allocation_status is now constant-time,
which directly benefits -Xcheck:jni.

A new gtest-based performance demonstration is included. It's not
really a test, in that it doesn't do any verification.  Rather, it
performs parallel iteration and reports total time, per-thread times,
and per-thread percentage of blocks processed.  This is done for a
variety of thread counts, to show the parallel speedup and load
balancing.  Running on my dual 6 core Xeon, I'm seeing more or less
linear speedup for up to 10 threads processing 1M OopStorage entries.

CR:
https://bugs.openjdk.java.net/browse/JDK-8200557

Webrev:
http://cr.openjdk.java.net/~kbarrett/8200557/open.00/

Testing:
jdk-tier{1-3}, hs-tier{1-5}, on all Oracle supported platforms