Move to elasticsearch-java 8

[altro3]

Removed the use of RestHighLevelClient and xcontent

Fixes #305

[altro3]

@graemerocher sorry, I don't understand what does it mean:

Execution failed for task ':elasticsearch:japiCmp'.
> A failure occurred while executing me.champeau.gradle.japicmp.JApiCmpWorkAction
   > Detected binary changes.
         - current: elasticsearch-5.0.0-SNAPSHOT.jar
         - baseline: micronaut-elasticsearch-4.2.0.jar.

How to fix it?

[graemerocher]

This is because the changes are binary incompatible with the previous release. To resolve this the changes have to accepted with a file like:

https://github.com/micronaut-projects/micronaut-core/blob/3.4.x/config/accepted-api-changes.json

You can run ./gradlew elasticsearch:japiCmp and open the report to see example JSON snippets that need to be included in this PR to accept the changes.

We have this in place to prevent breaking binary incompatibility in projects

[altro3]

@graemerocher thanks! Now all fixed

[graemerocher]

@timyates This module upgrade features a breaking major version upgrade so we would only be able to include in the BOM from Micronaut 4.0 onwards

[graemerocher]

/cc @sdelamo

[timyates]

@graemerocher should this be 4.0.x?

https://github.com/micronaut-projects/micronaut-elasticsearch/pull/320/files#diff-3d103fc7c312a3e136f88e81cef592424b8af2464c468116545c4d22d6edcf19R17

[graemerocher]

it should indeed

[timyates]

Other than the core branch version, it lgtm

I pushed a branch to get sonar to check it, and sonar likes it too