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

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.

Start by locating the transaction-sending path and how tm-load-test defines each send period. Compare uniform and random spreading as the two behaviors named in the issue, then verify that transactions are no longer emitted only at the beginning of each period and that the existing burst behavior remains supported.

Written by the indexing model from the issue text.