Add attribute filtering in GET /timespans

Enable attribute filtering by parsing "<key>_" and "<key>_like"
parametres into SQL JOIN queries.

Convert database queries in 'timespans' to esqueleto as required by the
use of advanced SQL queries, and add esqueleto >=2.0 as a dependency.

Signed-off-by: Stian Ellingsen <stian@plaimi.net>