Use symbols.exe to manage dump contents

At the moment we're saving uploaded dump files to our own blob storage. It's okay for a user base of 2, and to connect the dots, but there is going to be issues with this in the long run.

Namely,

• Non-unique file names just stomp on each other.
• Duplicate files, even with different names wastes space. Symbols.exe will calculate a hash and store files using that, as well as managing ref counts.