perf(833): Do not clone query plan during execution

[Centril]

Description of Changes

Fixes #833

Perf numbers on my machine:

Benchmarking incr-join: Collecting 100 samples in estimated 5.01
incr-join               time:   [5.0575 µs 5.0914 µs 5.1521 µs]
                        change: [-19.535% -18.714% -17.870%] (p = 0.00 < 0.05)
                        Performance has improved.

Benchmarking query-indexes-multi: Collecting 100 samples in esti
query-indexes-multi     time:   [1.0330 µs 1.0387 µs 1.0457 µs]
                        change: [-13.574% -12.820% -12.213%] (p = 0.00 < 0.05)
                        Performance has improved.

API and ABI breaking changes

None

Expected complexity level and risk

1

Testing

No changes semantically; should be covered by existing tests.

[joshua-spacetime]

Not sure what's going on here

full-join               time:   [282.30 µs 283.82 µs 286.06 µs]
                        change: [+11.033% +11.400% +11.830%] (p = 0.00 < 0.05)
                        Performance has regressed.

[joshua-spacetime]

I'm not sure what's causing the regression in full-join, but this is extra indirection.

[joshua-spacetime]

It's unclear where this regression is coming from at the moment.