Update testsuite with storagedriver interface changes

This change updates the testsuite to migrate to the new driver interface. This
includes the new Stat call, changes to int64 over uint64 and the changes to the
WriteStream signature. Several test cases have been added to vet
implementations against various assumptions.
Migrate filesystem driver to new storagedriver calls

The filesystem driver has been migrated to impleemnt the storagedriver
interface changes. Most interetingly, this provides a filesystem-based
implementation of the Stat driver call. With this comes some refactoring of
Reads and Write to be much simpler and more robust.

The IPC tests have been disabled to stability problems that we'll have to
troubleshoot at a later date.