Investigate Thread Safety Of SharedDataHolder::hashValid

[danieldresser-ie]

We noticed an issue that can be reproduced in Gaffer by removing the line tile->cachedTile->Object::hash(); from Display::channelData() and running InteractiveArnoldRenderPerformanceTest.testPerfWithBlur with GAFFER_HASHCACHE_MODE set to Checked. The validator reports hashes varying when they shouldn't, which John hypothesizes is due to the way that SharedDataHolder uses a non-threadsafe approach with the hashValid variable handling multiple accesses to the hash. We don't have hard evidence of this, but the fact that precomputing the hashes while holding a lock fixes it, strongly suggests this hypothesis is correct.

We should create a test case that demonstrates incorrect hashing behaviour under heavy thread load, introduce a spinlock to fix it properly, and then make sure that this doesn't impact overall performance too much of tools that rely on SharedDataHolder, like Gaffer.