SentryOutputWriter support

added a sentry output plugin which supports the Writer interface for shared UDP connections.

Things to note:
* Raven - the Sentry client is the one that specifies where the final messages get routed, so there is no Sentry DSN in the configstruct for this output plugin.  The writer has a map of host/port->Connection objects.  This is managed by decoding the DSN per message and creating a new UDP socket if needed and stuffing it into the map.

I've tested this against metlog-py/heka branch and Sentry 5.0.13