[cascading3] Adds a checkpoint when hashjoin is merged with one of its sides

[rubanm]

part of #1465

Fixes the outstanding issue from #1521 where merging a hashjoin with one of its own sides is disallowed by Cascading. This adds a Checkpoint to the join result as a workaround.

[info]   cascading.flow.planner.PlannerException: [Hfs["TextDelimited[[0]...][run() @ org.scalatest.tools.Framework.org$scalatest$tools$Framework$$runSuite(Framework.scala:466)]
  may not merge accumulated and streamed in same pipeline: Merge(_pipe_2+_pipe_3)[by: _pipe_2:[{1}:0] _pipe_3:[{1}:0]]
[info]   at cascading.flow.planner.rule.RuleExec.performAssertion(RuleExec.java:390)

https://github.com/cwensel/cascading/blob/wip-3.1/cascading-hadoop/src/main/shared/cascading/flow/hadoop/planner/rule/assertion/DualStreamedAccumulatedMergePipelineAssert.java

cc @johnynek @piyushnarang

[johnynek]

looks good to me. Would love to see this tested at Twitter where there are many hashjoins before a full release.

[piyushnarang]

rename p1 to hashJoinPipe and p2 to hashJoinOperandPipe (or something on those lines?). Tends to be hard to keep track of p1, p2..

[rubanm]

renamed

[piyushnarang]

minor var rename but looks good to me

[rubanm]

Merging this. Will be testing it on pilot jobs before a full release.