Support additional directives to force Verdict to use specific samples

[dongyoungy]

It would be good if a user can specify/force which samples to use to Verdict in a query like following:
"SELECT ... FROM .. WHERE .... USE VERDICT SAMPLE 'blah'"

Such feature could be useful for testing/profiling, especially when there are multiple samples available for Verdict.

[barzan]

@dongyoungy can you think more carefully about this and suggest a more specific syntax? the problem is that a query might have multiple tables in it and even the same table mentioned multiple time (e.g., a self-join), and so you need to be able to say which types/sizes of samples to use for which instance of which table. If you can suggest a concrete syntax, @boyuanmike might be able to add this feature. Unless you need this feature much sooner in which case you need to implement this yourself.