Optimize for large bundles with numerous globs

[snipervld]

At the moment, Asset.ExpandGlobs{:.c#} doesn't cache results of provider.GetAllFiles("/"), so in the following example (simplified example - in the real app, this bundle contains different globs at different nested levels):

builder
    .Services
    .AddWebOptimizer(
        pipeline =>
        {
            //...
            pipeline.AddJavaScriptBundle(
                "bundle1",
                "js/*",
                "js/*",
                "js/*",
                "js/*",
                "js/*",
                "js/*");
            //...
        },
        options =>
        {
            options.EnableTagHelperBundling = false;
        });

Asset.ExpandGlobs will get all files 6 times. It can be a problem, if the virtual files hierarchy is huge.

This PR caches the results of provider.GetAllFiles("/") per provider into HttpContext.Items dictionary.

Also, this fixes the case when many complex bundles are located on the same page, e.g.:

...
<script src="~/bundle1"></script>
<script src="~/bundle2"></script>
<script src="~/bundle3"></script>
...

Each of them will reuse the cached file hierarchy if possible.

P.S. As you can see, I've tested this with EnableTagHelperBundling = false. ScriptTagHelper and LinkTagHelper call Asset.ExpandGlobs on each execution, which resulted in 5 seconds of rendering for just the script tags in my app, instead of something like ~0.1 seconds. :)