distrib join through coord node

[mkhludnev]

https://issues.apache.org/jira/browse/SOLR-XXXXX

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[mkhludnev]

thread https://lists.apache.org/thread/4xq81fxn5zcx6qtwxzgvq72qm7ff4wtd

[endika-ravn]

#4186 There's a different behaviour (or at least seems to be) between coordinator nodes and data nodes that act as proxy. I have modified the tests so the coordinator node is explicitly started as a coordinator.

[dsmiley]

CC @patsonluk

[mkhludnev]

It seem like the code here #4184 forwards request to the data node. At least, it's worth to clarify "coordinator" expectation/behavior in the guide. Real trouble with coordinator-only join occurs at #4186