Change some internal async Task methods to be async ValueTask (dotnet/corefx#40527)

Change some internal async Task methods to be async ValueTask (dotnet/corefx#40527)

We have some internal and private `async Task` methods that are only ever `await`ed.  Today there's no benefit to making them `async ValueTask` methods, so we've kept them as `async Task`.  However, if we end up changing the implementation of `async ValueTask` to pool underlying objects, there becomes a potential benefit to using `async ValueTask` for these instead of `async Task`.  This PR changes those in a variety of libraries where we care more about performance and allocations.  There are likely a few more methods we'd want to convert based on profiling, but I believe this represents the bulk of them.

Commit migrated from https://github.com/dotnet/corefx/commit/97d9fb9d6c7e1d50f527e290ef8e18767dbc6bbf