Add an API call to regenerate all generated streams

[mitar]

Add an API call to regenerate all generated streams. This will be useful if we will improve implementation of operators (for example, add overflow parameters) and we want to recompute things.

It could be easily implemented: we just delete all derived stream datapoints, mark it as pending, and then call backprocess.

It is true though that this means datapoint values for existing stream will be changed. This is destroying the API contract we have (that datapoints can only be appended). Maybe it is better to not support that and simply require user to delete derived stream and add new one, with same parameters. Stream's ID will change so it will be clear that it is another stream with another datapoints. And for end-user things will work the same because end-user will query streams by query tags.

[mitar]

I think we should not support this but available easy cloning of streams (maybe just as a high-level API through Django manage command or something).