added capability to configure poolboy MaxOverflow parameter which riak_cores default to cero.

This is to test performance comparison between reusing a worker pool (what risk core does) as opposed to forcing every worker to be spawned and then killed. In our case we use a lot of memory so killing the process should be better than waiting for GC.

Signed-off-by: Alejandro M. Ramallo <alejandro.ramallo@leapsight.com>