try to add a better localhost resolver. See issue #44

A "Smarter" localhost resolver:

see issue #44 
Trying to avoid a lot of calls to java.net.InetAddress.getLocalHost()
which under AWS seem to trigger DNS resolving and have non-zero (~10ms) latency *per event*.
Usually, the hostname doesn't change so often to warrant a real query.

A real call to java.net.InetAddress.getLocalHost().getHostName()
is made only if:
1) the refresh interval has passed (=result is stale)
AND
2) no env vars that identify the hostname are found