Perform merging of select queries by constructing and executing query plan

[cswinter]

There is one remaining place during query execution which is performed "manually" rather than by constructing and executing a query plan:

LocustDB/src/engine/execution/batch_merging.rs

Line 247 in 38ba86d

// TODO(clemens): make this work for differently aliased columns (need to send through query planner)

Mostly this just requires moving the Data::append_all function into a VectorOperator and corresponding QueryPlan variant and then constructing a query plan in the same fashion as all the other cases for merging batches.