Replace linear pacer with stepped pacer

[cjonas9]

What

Replaces Blaster's linear pacer with a stepped pacer. Endpoint blasts with a ramp up phase will now step through various RPS levels every --step-interval seconds (default of 5s, but whole number multiples of 5s are also accepted).
For added context, Blaster currently supports a linear and constant pacer (i.e. functions that determine the request-sending patterns, the former increasing RPS from 1 to target_rps over --ramp-up time, and the latter sending a constant RPS of target_rps for the duration of the run).

Why

The linear/constant pacer suffers from the following limitations:

• the target RPS displayed in live logs is an optimistic estimate of the RPS (the logs display the current RPS at five second intervals, but the statistics are for the previous responses, which were assuredly made when the RPS was less than it currently is).
• not straightforward to determine RPS level where the RPC buckles at if RPS is constantly being increased.
  The stepped pacer fixes this by stepping through RPS levels to see how an RPC performs when being hit with some set amount of load.

Known Limitations

N/A, other than removing the linear pacer functionality.

[Shaptic]

Coupla comments ⬇️ but otherwise LGTM!