item proxy index strategy, locate table segment

[randysecrist]

locate and store new items in their correct segment

https://github.com/basho-labs/rinamo/blob/master/src/rinamo_idx_item_proxies.erl#L13-L15

https://github.com/basho-labs/rinamo/blob/master/src/rinamo_idx_item_proxies.erl#L38-L40