remove legacy formatted indexes on write until per-partition fixed flag set

in order to safely migrate indexes, until the fold that rewrites the indexes in the
new format completes, all riak_object puts/deletes must delete the legacy index
format in addition to the new format

this commit also adds the ability to write a per-partition flag to the leveldb backend.
On startup, the backend attempts to read this flag and uses it to prevent the extra
deletions once they are no longer necessary

Additionally, migrate to uwiger/sext from esl/sext because the format is now the
official repository