Make ESTIMATED_QUERY_TIME_S user-configurable

[gabotechs]

Follow-up from #432 (thread).

gather_runtime_statistics in src/coordinator/prepare_dynamic_plan.rs hardcodes const ESTIMATED_QUERY_TIME_S: usize = 10, used to estimate bytes-per-second flowing through a stage.

Probably this estimation is better provided by people, still not sure if as a simple config option, or maybe a method in the TaskEstimator trait that receives the plan as an input.

[gabotechs]

Dropped in:

• Drop velocity-based estimation in favor of driver path completion estimation #552