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,

hustle/src/operators/select.cc

Line 95 in df14c96

return Filter(block, node->predicate_);

Sample ref for multithreaded implementation,

hustle/src/operators/select.cc

Line 55 in df14c96

ctx->spawnTask(CreateTaskChain(