WA34: WebAPI: ReflectedHttpActionDescriptor: Cache repeated calls to MethodInfo.GetParameters.

In WebAPI scenarios involving paramters, there are repeated calls to extract the same set of paramters on the long lifetime ReflectedHttpActionDescriptor. This changes caches them and will prevent one array allocation per request and should improve throughput by 0.3-0.5% depending on the scenario.
DCN02: WebAPI: ParseMediaTypeHeaderValue optimization.

In all WebAPI scenarios ParseMediaTypeHeaderValue is instantiated many times per request during the negotiation process and is used inside sorting and comparison operations. The change removes the 3 heap allocations per instantiation and optimizes the CPU path. Should improve high throughput scenarios by about 1%.