Pr/fix unit tests memory leaks (#1795)

* 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.

AsyncDisplayKitTests/ASMultiplexImageNodeTests.m

@@ -90,8 +90,22 @@ static BOOL ASRunRunLoopUntilBlockIsTrue(BOOL (^block)())
 {
   [super setUp];
 
-  _mockCache = [OCMockObject mockForProtocol:@protocol(ASImageCacheProtocol)];
-  _mockDownloader = [OCMockObject mockForProtocol:@protocol(ASImageDownloaderProtocol)];
+  _mockCache = [[OCMockObject mockForProtocol:@protocol(ASImageCacheProtocol)] retain];
+  _mockDownloader = [[OCMockObject mockForProtocol:@protocol(ASImageDownloaderProtocol)] retain];
+}
+
+- (void)tearDown
+{
+  if(_mockCache) {
+    [_mockCache release];
+    _mockCache = nil;
+  }
+  if(_mockDownloader) {
+    [_mockDownloader release];
+    _mockDownloader = nil;
+  }
+
+  [super tearDown];
 }
 
 - (void)testDataSourceImageMethod