Dumping the metadata file

@agorrod, I tried to dump a metadata file today using `wt dump file:WiredTiger.wt` and I couldn't, wt kept insisting the object didn't exist.

I eventually figured out that `metadata:` was a magic string (is that documented anywhere?), but even knowing that I couldn't find a way to convince wt to display the contents of the metadata file.   Maybe I'm just missing something, and all we need to do is document the `metadata:` string?

Anyway, by the time I figured out what was going on, I had these changes done, so if there isn't a way to dump the metadata file, maybe this is the right fix?

It removes the magic of `metadata:`, and just matches against the metadata file's URI.