Add functionality to retrieve elements count

[mihaidma]

There is no operation to find out the total number of elements that a query returns without retrieving the element list from the server.
The current store-test version is 1.1.0 (aka the current spec version), most of the stores currently implement 1.0.0.

If this new feature request is accepted we should add in 1.2.0 a count functionality test. In time stores will implement it when they adhere to 1.2.0 specs.

@rjrodger @mirceaalexandru what do you think? Is there is a reason for not implementing this functionality until now?

[akrsmv]

reasoning on that request is here

[mirceaalexandru]

I also think we should have this feature, don't know for sure if all DB engines have support for it so some of our stores might not implement this and we should specify this in documentation.

The count$ should accept same parameters as list$, not considering limit & skip & order.