…ioning is not optimal. (#4431)

## What

Force the "preserved" side of a semi/anti join to be partitioned, even
if is smaller than the non-preserved side(s).

## Why

Currently, when the joinscan would not be able to use optimal
partitioning for a semi/anti join, it refuses to run, with:
> JoinScan not used: SEMI JOIN requires the left side to be the largest
source

This is reasonable, but the problem is that this is based on estimates
that change based on the data distribution and WHERE clause selectivity,
rather than based on the shape of the query itself. That means that if
you've written a query that uses `pdb.score` and tested it on a
particular set of data, it might begin failing (with an "Unsupported
query shape" error) on a different set of data.

## How

Add and use `with_forced_partitioning`.

(cherry picked from commit 0b20263)