Added sql parsing and execution classes for the DISTINCT predicate.

Summary of the changes / Why this improves CrateDB

Implement the IS DISTINCT FROM predicate. It is just like <> but handles NULL values differently.
TODO: arrays and possibly type check.
The predicate is implemented as a Function. Please review if this is the right choice.

Checklist

• Added an entry in the latest docs/appendices/release-notes/<x.y.0>.rst for user facing changes
• Updated documentation & sql_features table for user facing changes
• Touched code is covered by tests
• [x ] CLA is signed
• This does not contain breaking changes, or if it does:
  • It is released within a major release
  • It is recorded in the latest docs/appendices/release-notes/<x.y.0>.rst
  • It was marked as deprecated in an earlier release if possible
  • You've thought about the consequences and other components are adapted
    (E.g. AdminUI)