all: make coalesce a logical node

[MichaHoffmann]

Making "coalesce" a logical node, allows us to move it around in the execution tree and eventually coalesce later and evaluate more of the query concurrently. This will allow us to subsequently treat local execution almost like remote execution, so that we can compute aggregations in shards.

[fpetkovski]

This change potentially modifies the contract with the scanner. I would like to test it with our downstream project this week before we merge it.