ReadWriteLock upgradeToWrite fix for simultaneous upgrades.

[accelerated]

Description

If ReadWriteLock::upgradeToWrite() was called and there were more than one simultaneous upgrades occurring, the function would block indefinitely. This was caused by the underlying loop calling tryUpgradeToLock() not keeping upgrade state during iterations.
This fix introduces a new overload ReadWriteSpinlock::tryUpgradeToWrite(bool& pendingUpgrade) which should be used if calling this in a loop.

Testing performed
The previous tests only covered calling ReadWriteSpinlock::upgradeToWrite() which worked properly because it was not invoked inside a loop. The ReadWriteMutex::upgradeToWrite() calls it in a loop which was never actually tested. As a validation, running the new added test on the previous master branch blocks indefinitely which proves the fix is working correctly.

Signed-off-by: Alexander Damian adamian@bloomberg.net

[arosenzweig3]

Looks good.