[runner][worker] Limit QPS

[at15]

Besides limit by time/points, we can add extra constraints like limit QPS, it is different from firs two because those two determines the termination of workload, QPS controls the fastest speed each worker thread should be.

Also from YCSB about latency when QPS is limited

if you specify a target of 10 operations per second (and a single thread) then the Client will only execute an operation every 100 milliseconds. If the operation takes 12 milliseconds, then the client will wait for an additional 88 milliseconds before trying the next operation. However, the reported latency will not include this wait time; a latency of 12 milliseconds, not 100, will be reported.