Wrap the call to update_index_info in global:trans
Wrap the call to update_index_info in global:trans

This fixes a race condition that was causing stats corruption if two
processes tried to update the data for the same index at the same time.

As detailed in the code comments, this may not have the best possible
performance characteristics, but it seems to be good enough and is a
very easy workaround for the problems we found.

For future reference, we originally found and reproduced this bug using
yz_aae_test. It didn't always trigger the race, but adding a short
timer:sleep call in between the lookup and insert calls made it very
easy to catch.

andytill/riak_kv