Docs discovery: Implement basic working version

[islamaliev]

We want to be able to discover documents from the network, i.e. request potentially unknown to local node documents from the network.

For this we need to create an explicit GQL query like discover_User that takes regular filter as well as limit and send them to the peers. The query should return the list of docIDs. We don't want to run aggregate queries on them.

The first version doesn't need to care about network efficiency. This will be handled in follow up tasks.