HttpClient should static and or lazy

[BryanWilhite]

depends on https://github.com/BryanWilhite/SonghayCore/issues/18

[BryanWilhite]

the subtlety here is that my intention is that all the feed downloads share the same HttpClient this means the downloads cannot run in parallel (so no Task.WhenAll / Task.WaitAll):

internal void DownloadFeeds(FeedsMetadata meta, string rootDirectory)
{
    if (string.IsNullOrEmpty(meta.FeedsDirectory))
        throw new NullReferenceException("The expected feeds directory is not configured.");

    foreach (var feed in meta.Feeds)
        this.DownloadFeedAsync(feed, rootDirectory).GetAwaiter().GetResult();
}

the explicit use of foreach with GetAwaiter().GetResult() shows the intent to run several async tasks sequentially

• https://stackoverflow.com/questions/30306497/sequential-version-of-task-whenall
• https://stackoverflow.com/questions/6123406/waitall-vs-whenall

• https://stackoverflow.com/questions/39007006/is-getawaiter-getresult-safe-for-general-use

• https://github.com/dotnet/corefx/issues/32507

the above caveats suggest that i should not use GetAwaiter().GetResult() in, say, Azure Functions