Support consuming multiple tokens from single store in single step

[raulmonti]

It is unfortunate that I need to add new tasks just to be able to consume another token from the same store. It would be helpful to be able to consume more than one token from the same store in the same task. This is only useful of course if I am able to distinguish the tokens I consume. So we should find a way to name them. Currently the single consumed token is named after the datastore from where it comes from. Here is an alternative for this:

Lets say the datastore is called data_store, then we may be able to consume n tokens and refer to them by data_store_1 to data_store_n.

Lets propose some other alternatives. Also, we should propose a way to indicate the cardinality of the relation (how many tokens I consume in task T from datastore D).