Fix race condition and spelling error in metadata reader

2 separate fixes. See individual commit messages for more details.

1) Race condition revealed by http://roslyn.codeplex.com/workitem/299
2) Spelling error in rarely used pre-release API name




I prefer the black box approach here. Rather than have the test get tangled with the internals of GetMetadataReader, we simply stress the scenario of doing it in parallel 1000 times. This is not a unit test, but a stress test, which is why the [Fact] is commented out with a TODO. We'll figure out soon how to run these outer loop tests regularly (but won't slow down every build with them). Once that happens, the probability that a regression would be left unfound indefinitely is negligible.

Without the fix, the test fails for me 20 consecutive times and passes 20 consecutive times. Theoretically, 

Yes, theoretically, it is possible for the test to pass with the fix, but the probability is negligible. If this were running regularly (which it will once we have our stress) the