Add optimizer rule to move a limit beneath a union

[mkleen]

Summary of the changes / Why this improves CrateDB

This adds the optimizer rule optimizer_move_limit_beneath_union which reduces the number of intermediate rows created when a LIMIT clause is used in combination with a UNION operator, by moving the limit beneath the union operator e.g.:

 SELECT a FROM t1 UNION ALL SELECT b FROM t2 LIMIT 10;

has the logical plan:

Limit[10;0]
  └ Union[a, b]
     ├ Collect[doc.t1 | [a] | true]
     └ Collect[doc.t2 | [b] | true]

becomes after the rule application:

Limit[10;0]
  └ Union[a, b]
    ├ Limit[10;0]
    │  └ Collect[doc.t1 | [a] | true]
    └ Limit[10;0]
       └ Collect[doc.t2 | [b] | true]		

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
• 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)

[matriv]

Thank you! LGTM