Allow routing to different hosts based on event attributes

[yaauie]

To summarise #44 and elastic/logstash#9233, it is surprising that the "host" property of this plugin is not interpolated on a per-event basis.

Doing so would require:

• interpolating the @host per-event
• multiple persistent connections to external syslog
• connection-management (e.g., reaping expired connections, thread-safe connection creation, etc.)