Increase directory lookup speed

This improves lookup speed in a number of ways. The mutex locking has
been removed, which I believe is the primary culprit of the increased
lookup times. Writing new listings only happens in a single thread, so
the mutex was an unnecessary precaution.

Additionally, the listings are now hashed by service name for the lookup
method, removing all iteration from the lookup process (except perhaps
on a hash collision).
Cleanup the specs
Benchmark lookup performance
Only lookup defined services in the benchmark
Randomize benchmark servers ttls'
Merge pull request #114 from devin-c/fast_directory_lookups

Increase directory lookup speed