option to auto adjust chunk size

Similar to what pt-osc does, give the option to adjust chunk size dynamically to try to make chunks run in exactly --chunk-time seconds. In our testing this can make a big difference in reducing the time it takes to run a schema change.
From pt-osc doc:
The tool tracks the copy rate (rows per second) and adjusts the chunk size after each data-copy query, so that the next query takes this amount of time (in seconds) to execute. It keeps an exponentially decaying moving average of queries per second, so that if the server’s performance changes due to changes in server load, the tool adapts quickly.