Improving unmanaged extensions docs

* Example of streaming back the response
* Example of using a single ExecutionEngine
* Reordering the explanation of the basic example
Improvements to unmanged extention and neo4j-harness docs.
Merge pull request #3625 from nawroth/2.1-unex-docs

Improving docs for unmanaged extensions and neo4j-harness
Merge remote-tracking branch 'upstream/2.1-maint'

Conflicts:
	community/server-examples/LICENSES.txt
	community/server-examples/NOTICE.txt
	community/server-examples/src/docs/dev/server-unmanaged-extensions.asciidoc
Update unmanged extension docs for 2.2.