Switch tombstones to a new data format

[g2p]

From @g2p on September 25, 2018 9:35

Discussed in #51.
At the moment tombstones are empty values + a mount option, but this should be switched to a superblock option to get consistent semantics across mounts.
While we are adding a superblock flag, also add support for prefixing values with a type when the flag is enabled, so that tombstones are distinct from empty values, and that upsert data types can be added later.

Copied from original issue: g2p/storage-unikernel#63