Add backpressure mechanism

[jamebs]

What?

We would like to add an ability to limit the number of processed and blocked results.

Why?

This will eliminate overflow error when the first group is processing much faster than the second one.

How?

By adding a new group's option which will set upper limit of blocked results. When the group reaches the limit, it must wait until the number of blocked results is reduced to begin processing a new request. The option/mechanism will be disabled by default