Add format downgrade script [JIRA: RIAK-1920]

[engelsanchez]

This script downgrades Bitcask data files to the format used before version 1.7.0 (to ship in Riak 2.0). In that version, a more compact encoding is used for key values, and also 2 new types of tombstones are introduced.

Simply pass the script's location to escript and the root directory for all bitcask files that need to be reformatted:

$ escript downgrade_bitcask.erl /my/riak/data/bitcask

[bearcage]

Verified for upgrade/downgrade cycles on 1.4.10 and 1.3.2 with data on the old and new sides coming through fine. :thumbsup:/+1

[engelsanchez]

@borshop merge

[DSomogyi]

create jira issue