New strategy based on documents modification date

Hello.

I tried to make a new strategy for the jdbc river, based on modification date. The river indexes last modified objects and stores the last modification date, last execution date and river status in _custom object in the river's index.

I add a new Merger (ComplexMerger) which manages more cases (arrays of objects...).
To avoid limitation of SQL databases (30 characters in column name), I used mapping settings instead of names in SQL request (parameter "mapping").

I add strategy parameter to define the appropriate one to use in the river (timebasis, rivertable, standard (empty)).

I disabled the mysql tests cause I don't have a mysql installed. Instead, I used an embedded database, HSQLDB, and you can also use a H2 database.

I add tests with embedded ElasticSearch.