Followups to #134967:

• PipelineBreaker interface and ExecutesOn are very close and intersect a lot. We should probably refine that and have either one interface or separate interfaces with well-defined purposes.
• Some classes are now "sometimes PipelineBreaker" - like Limit - but the existing interface does not allow to express that.
• Figure out how PushDownAndCombineLimits should deal with Joins, remote or local. Right now limit pushed under join forces that to the coordinator side. This may not be ideal.