Refactor Foolscap code so it's separate from non-Foolscap code in storage API

[itamarst]

Right now Foolscap implementation is heavily tied in to StorageServer?, BucketWriter?, and BucketReader?. Sometimes the business logic is separate method, sometimes not.

This means it's harder to see which parts are Foolscap-specific, it's harder to make the type checker happy due to RIStorageServer interfaces and friends (which specify foo() but need remote_foo()), and it makes ripping out Foolscap later on harder.

So, much like HTTP is a separate class wrapping the underlying StorageServer?, Foolscap should also be separate classes wrapping the underlying StorageServer? etc..