Add flag for controlling percentile of commit time used in scheduler

In OAK-4732 the 50th percentile of the last 1000 commits is used as wait time before returning the current root. In order to parametrise the value of the percentile, it would be nice to have a new feature flag, e.g. oak.scheduler.head.lockWaitPercentile. Setting it to 0 would basically disable this. Setting it to a different value might be interesting in future experiments.