When decoding data from mmaped blocks, we would like to retrieve
a string backed by the mmaped region. As the underlying byte slice
never changes, this is safe.
* Expose Stone as it is used in an exported method.
* Move from tombstoneReader to []Stone for the same reason as above.
* Make WAL reading a little cleaner.
Signed-off-by: Goutham Veeramachaneni <cs14btech11014@iith.ac.in>