apmbench: simplify the warmup logic by providing a `-warmup-time` flag

[marclop]

Description

Currently, apmbench uses an event-driven warmup which relies on a dynamically calculated timeout, all of which overly complicates the warm up logic. Instead, it would be better to switch to a time-based warmup approach and cause stop all the goroutines after the warmup has elapsed.

Introducing a new -warmup-time flag and removing -warmup-events will provide a more consistent and predictable behavior.