Increase spark.akka.askTimeout default to 30 seconds

In experimental clusters we've observed that a 10 second timeout was
insufficient, despite having a low number of nodes and relatively small
workload (16 nodes, <1.5 TB data). This would cause an entire job to fail
at the beginning of the reduce phase.
There is no particular reason for this value to be small as a timeout should
only occur in an exceptional situation.

Also centralized the reading of spark.akka.askTimeout to AkkaUtils (surely
this can later be cleaned up to use Typesafe).

Finally, deleted some lurking implicits. If anyone can think of a reason they
should still be there, please let me know.