Extract XContent from the elasticsearch server

[nik9000]

One day we'd like the high level rest client not to depend on Elasticsearch's server code. As a first step we're going to extract Elasticsearch's xcontent classes into a jar without dependencies.

• Replace xcontent's explicit dependencies on Log4J with something pluggable and have the Elasticsearch server plug Log4J into it (@dakrone)
• Separate xcontent from Lucene (@dakrone)
• Separate xcontent from other parts of Elasticsearch (like commons) (@dakrone)
• Build the jar

[nik9000]

#28449 and #28488 started this process. It'll take a few more PRs to finish it.

[nik9000]

As is #28503.