add migrate-index.sh for simplified reindexing.

migrate-index.sh is a new script to automate index migration, given ES URL, index alias (pointing to the "live" index), and name of destination physical index.

test-migrate and gen-1k-threads are used to test this.  the logic is explained in test-migrate.

running this repeatedly on devstack gives inconsistent results.   sometimes it passes; sometimes we come up with 1 or 2 too many documents (missed deletes), and sometimes we come up short by some number of documents (the most concerning case) - anywhere from a few to a few dozen.

i've also noticed that sometimes incremental-copy-index returns "-Infinity" for the max dates, which suggests either the query is non-deterministic, or maybe that i'm just using this wrong. 

@e0d @gwprice