Perf stresstest workload

[anjagruenheid]

Custom workload that executes performance stress tests by a) increasing the number of joins present in a base query and/or b) query padding. These augmented tasks are executed concurrently by duplicating sessions (if specified by the user). This PR introduces the end-to-end workflow for this workload pattern and addresses the following fixes to the codebase:

1. SQLParser now outputs strings rather than StatementExecs for a cleaner separation of functionality.
2. Event identifiers have been modified to essentially showcase the hierarchy. For example, the first statement in file a, part of task t and executed in session 1 is now identified as 1;t;a;0.
3. Adds a layer of abstraction for custom task execution to be consistent with the argument implementation.

** This PR does not yet contain sufficient test cases and will not be committed until those are added. **

[jcamachor]

@anjagruenheid , could we split the changes that refactor/move CustomTaskExecutor into a new PR that we could try to merge as soon as possible (since maybe other aspects of this PR need further discussion)? I'm working on #139 and there will be some merge conflicts otherwise. Thanks