adaptive concurrency: Allow configurable jitter to minRTT calculation windows

[tonya11en]

The adaptive concurrency control filter periodically recalculates the expected minimum round-trip time for requests. This recalculation drops the allowed concurrency limit to 1 while it measures this value, leading to a drop in success rate.

This effect can be mitigated using a previous_hosts retry predicate. However, to prevent synchronization of these minRTT calculation windows, it will be necessary to introduce a jitter parameter (allowing us to specify the duration + or - some delta) to the filter config.

For simplicity, this could be percentage based.

Relates to #7789