[planner] support the distrubuted plan fragment

[BohuTANG]

[summary]

For now, all the pipeline build based on the single fragment, if we have a query:

SELECT SUM(number) from system.numbers_mt

Thie pipelines building like:

  └─ AggregateFinalTransform × 1 processor
    └─ Merge (AggregatePartialTransform × 8 processors) to (MergeProcessor × 1)
      └─ AggregatePartialTransform × 8 processors
        └─ SourceTransform × 8 processors  

All 8-way parallel is executed on the local, so we need the plan can split to many fragments and some of them can be executed on the remote site:

  └─ AggregateFinalTransform × 1 processor
    └─ Merge (AggregatePartialTransform × 8 processors) to (MergeProcessor × 1)
      └─ AggregatePartialTransform@remote1 × 2 processors
        └─ SourceTransform@remote1 × 2 processors  
      └─ AggregatePartialTransform@remote2 × 2 processors
        └─ SourceTransform@remote2 × 2 processors  
      └─ AggregatePartialTransform@local × 4 processors
        └─ SourceTransform@local × 4 processors  

Local and remote stream is connected by the ExchangeStream port.