Support Parallel Scan

[zhangh43]

Konstantin  wrote:
Parallel scan is scattering pages between parallel workers.
To fill VectorTupleSlot with data you may need more than one page (unless you make a decision that it can fetch tuples only from single page).
So it should be somehow take in account specific of parallel search.
Also there is special nodes for parallel search so if we want to provide parallel execution for vectorized operations we need also to substitute this nodes with
custom nodes.

[yjhjstz]

looking forward.