Opt4c: MVC: Fix caching bug in provider collections.

The change to cache dependencies on several Collection<T> implementations were missing the ClearItems override. Added unit tests for all the mutators.
Opt23B: MVC: Simplified pattern for lazy initialization of PrefixContainer.

It has been determined that a simplified pattern for lazy initialization of is thread safe on all platforms.
WA21B: WebAPI: Updates to WA21, removing Lazy<T> from NameValuePairsValueProvider from further review.
WA22: WebAPI: DefaultServices: Improve cache lookup performance.

WebAPI Async scenarios show an unexpectedly high inclusive CPU% in DefaultServices.GetService of 2.5% and its read lock is the 2nd highest source of contentions at 16.77%. This change reduces the inclusive time to 0.6% and eliminates the contentions.
WA25: WebAPI: Avoid RunSynchronously.

WebAPI scenarios allocate many delegates and closures. This change avoids using the RunSynchronously helper on WebAPI requests which saves 4 object allocations and two delegate invocations per request.