Reduce warnings emitted by javac in Java 7/Java 8 [LUCENE-6740]

This is a overview issue about improvements to reduce warnings during the build. Since we moved trunk to Java 8 and branch_5x to Java 7, there were introduced some additional warnings, mostly in generated code.

ANTLR4 does automatically add the needed `@SuppressWarnings`, but jflex does not. There are also some rawtypes warnings, which changed in Java 7 (before Java 7 there were only "unsafe" warnings, now you need both.

I enabled the Warnings tracker in Policeman Jenkins, we can therefore very easily check the warnings: e.g. on trunk: http://jenkins.thetaphi.de/job/Lucene-Solr-trunk-Linux/ (see diagram on the right), you can click on it, e.g. http://jenkins.thetaphi.de/job/Lucene-Solr-trunk-Linux/warnings4

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Migrated from [LUCENE-6740](https://issues.apache.org/jira/browse/LUCENE-6740) by Uwe Schindler (@uschindler), updated May 09 2016
Linked issues:
- #7799