Load balancing support

[praseodym]

A common use case of the TCP output plugin is to send events to another Logstash instance running the TCP input plugin. For (horizontal) scalability, a load balancer can be placed in front of multiple Logstash instances on the receiving end. However, this output plugin will only use a single connection to send all events. If a single instance produces many events, this means that the receiving end will have to be powerful enough to process all these events. I propose to add support for load balancing over multiple connections (to multiple hosts, #11, or to a single host running a loadbalancer) so that the processing load can be split by multiple receiving Logstash instances.