* Fix some concurrency problems detected by Xcode 8's new Thread Sanitizer.
Some of these changes are arguably just to silence the warnings from Thread Sanitizer.
* Fix several memory leaks in the unit tests.
A number of the unit test source files are compield with `-fno-objc-arc`. This was clearly overlooked when writing several of the unit tests.
Fixed by (mostly) switching to use of `-autorelease` for the problem code.
NOTE: This commit doesn't fix all the memory leaks found. There's still at least one leak in `-[ASDisplayNodeTests testSetNeedsDataFetchImmediateState]`, and several leaks in `ASBasicImageDownloader.mm`. I wasn't able to find a trivial cause to these, unfortunately.
These have been superseded by -[ASDisplayNode initWithViewBlock:] and
-[ASDisplayNode initWithLayerBlock:], respectively -- the new API allows
for custom initialisers, but does not support asynchronous display.
The old initialisers are still available in ASDisplayNodeInternal.h, for
internal subclasses and daring adventurers.
