Change concurrency level after initialization

[slumbi]

Currently it is possible to decrease the concurrency of the queue, and it behaves as expected.
Increasing it however does not work, because each time a task finishes , only one new task gets executed instead of one task per available slot ( concurrency-busyCount )

This feature would allow to dynamically change processing speed, and even pause the jobs for a period of time.

[jjrv]

I guess TaskQueue should have a setConcurrency method that also calls next when increasing it, to immediately start new eligible tasks before previous ones finish.

Then next could have a loop like:

while(this.busyCount < this.concurrency) { ... }