Support spreading transaction load across time instead of bursts every send period

[thanethomson]

At present, tm-load-test sends transactions in bursts at the beginning of each send period (usually each second). We should also support spreading this transaction load across each second, either uniformly or randomly.

[williambanfield]

It's not clear to me this actually needs to be done in the tool, since users could alternatively set a smaller period with an equivalently smaller batch size whose product achieves the same throughput. For example, to achieve 100 tx / sec, you can either use 1 second as the period and 100 as the size or .5 seconds as the period and 50 as the batch size.