Add native support for SQL and EQL queries

[Luegg]

For benchmarking SQL and EQL queries we're currently using custom runners (see here for EQL and here for SQL).

As @danielmitterdorfer pointed out, there is some potential to improve the benchmarks by discarding the response instead of parsing it (as usually done by the ES client).

[Luegg]

Additionally to a native "sql" operation type it would also be very useful to have a "sql-scroll" operation analogous to scroll-search.

[michaelbaamonde]

Thanks for raising this @Luegg. We'd definitely like to limit the need for custom runners as much as possible, especially in our default tracks. I'm working in this general area of the Rally code right now, so I'm assigning myself, but if someone else wants to give it a shot, please feel free.