I/O scheduler: make "Tau" configurable

[vladzcloudius]

"Tau" is a parameter of an I/O scheduler which value can significantly improve the scheduler performance or make it worse depending on a specific workload.

In particular "tau" is a factor that defines the time a scheduling class that was inactive may occupy I/O after it becomes active: the greater the "tau" value the longer it will occupy it.

However if "tau" is too short that a bursty low priority class that comes and goes may eventually have more runtime than the higher priority class.

It makes sense to allow configuring it while having some reasonable default.

[vladzcloudius]

@xemul FYI