OAK-9211: elastic dynamic boost

[fabriziofortino]

This PR introduces dynamic boost for Elastic. Although some logic used in Lucene has been refactored and moved in FulltextDocumentMaker, the elastic version differs on the following aspects:

• the boosting is performed at query time
• structurally the docs are stored as nested docs

(lucene)
dynamicBoost.red=1 (boost=9)
dynamicBoost.blue=1 (boost=2)

(elastic)
dynamicBoost [
  {
    token="red"
    boost=9
  },
  {
    token="blue"
    boost=2
  }
]

The main differences are: the tokens are stored as analyzed values (instead of field names) and the boost is a value. This value is used at query time with a script score query.

The benefits of this implementation are:

• less sparse documents leading to better disk space usage (https://issues.apache.org/jira/browse/LUCENE-6863)
• better relevance since the boost query is not an exact match anymore but full-text analyzed