Multithreaded Predicate execution

[srsuryadev]

Currently, in the select operator, we do multithreading between different batches (i.e a set of blocks). In this task, we will experiment and see if we are getting speed-up by doing much fine multithreaded execution.

• [ ] Do multithreaded execution of the leaf nodes (i.e comparison operator execution) in the predicate tree.
• [ ] Have this feature togglable so that it can enabled and disabled easily using macros.
• [ ] Compare the SSB benchmark performance by enabling it and disabling it.
• [ ] Add further robust unit tests to the Select operators

Ref: Comparison operator call, https://github.com/UWHustle/hustle/blob/df14c968e91e64babf15478268eed517fa82768b/src/operators/select.cc#L95 Sample ref for multithreaded implementation, https://github.com/UWHustle/hustle/blob/df14c968e91e64babf15478268eed517fa82768b/src/operators/select.cc#L55