Support remote exchanges in LocalQueryRunner

[martint]

We currently have two mechanisms for running queries in unit tests: LocalQueryRunner and DistributedQueryRunner. LocalQueryRunner is faster to set up as it doesn't involve running a full server & exchange clients, but can only execute queries with a single fragment. The downside is that it's not suitable for testing behaviors that only manifest when the optimizer inserts remote exchanges into the plan. As a result, all the tests that rely on LocalQueryRunner are missing opportunities to catch correctness issues due to bad plans

We should:

• Add the ability to set up the ExchangeOperator with a custom ExchangeClient that's wired directly to the OutputBuffer of the upstream fragment.
• Update LocalQueryRunner to support plans with multiple fragments.
• Remove the forceSingleNode mode from PlanOptimizers.

[martint]

These are examples of queries that only fails when remote exchanges are involved, so it's not possible to test them with LocalQueryRunner (and therefore, in the style of io.prestosql.sql.query.TestXXX): https://github.com/prestosql/presto/pull/262#discussion_r258687795

[findepi]

Another unique feature of LocalQueryRunner is that it's single-threaded. Are you going to change this too?

(This has obvious downsides but also has an upside -- if you make one of the Join operators block on the other during execution, the LocalQueryRunner will (rightfully) deadlock.)

[dain]

Another unique feature of LocalQueryRunner is that it's single-threaded. Are you going to change this too?

I don't think we should change that. The distributed tests are there to catch those kinds of issues. BTW, the local query runner "works around" the deadlock problem by just ignoring all of the blocked signals.