Introduce conduits as potential replacement of cursors

[VictorDenisov]

Conduits are more natural abstraction for haskell.

[Yuras]

Can it be done in a separate package?

Reasons not to include it into the main package: conduits is a new dependency; other people may prefer other strraming solutions; cursors are dead simple compared to streaming libraries, newcommers don't have to study one to use cursors.

[gregwebs]

I would not worry about the additional dependency of plain conduits: it is a very light-weight package in terms of dependencies it brings in.

persistent-mongoDB has an implementation of a conduit source already:
https://github.com/yesodweb/persistent/blob/master/persistent-mongoDB/Database/Persist/MongoDB.hs#L728
https://github.com/yesodweb/persistent/blob/master/persistent-mongoDB/Database/Persist/MongoDB.hs#L756

So it might be possible to bring that in to the package and present both the existing cursor interface and a conduit interface.

[VictorDenisov]

I'm actually thinking about not plain conduits, but concurrent conduits. At the moment working with cursors is a little cumbersome. So, probably removing cursors completely is not a right idea. I need to think about what @Yuras said. Maybe it makes sense to ship something like mongodb-extras where we provide convenience functions for working with mongodb.