Add hackSql Invoker/Executor method

for replacing the sql generated by Slick's query compiler by hand-written sql

Slick's type-safe API becomes unusable, when the compiler generates queries the optimizer of the targeted db can't efficiently deal with. A known case are nested subqueries, where MySQL <= 5.5 stops may stop using indices and do unnecessary table scans. This implements a viable workaround, which allows to replace the generated SQL by hand-written SQL.

Replaces suggested hack in #807.

review by @szeiger