provide a default value for local host name lookups
Merge pull request #304 from pyr/issue-233

provide a default value for local host name lookups
Threadpool executor service: better queue depth estimate.

Now ignores tasks actively executing.
Merge branch 'master' of github.com:aphyr/riemann
Adjust threadpoolservice queue/pool parameters.

Experimentation with async-queue suggests these will better meet the
default needs of users for IO-bound pools.