cassandra: speed++
cassandra: speed++

- use libev reactor when we can
- increase parallelism slightly
- disable metrics, they take a lot of time to compute
- executor_threads wasn't what we think it was

Thib17/biggraphite,criteo/biggraphite,iksaif/biggraphite,criteo/biggraphite,iksaif/biggraphite,criteo/biggraphite,criteo/biggraphite,Thib17/biggraphite,iksaif/biggraphite,Thib17/biggraphite,iksaif/biggraphite,dpanth3r/biggraphite,Thib17/biggraphite