nsq_to_http: add host pooling option for handling upstreams

`nsq_to_http` currently supports only two (naive) strategies when the user specifies more than one upstream to write to, random and round robin.

the problem with both of these is that one bad host can significantly impact throughput because it is continually tried.

by "host pooling" them, we can exponentially backoff bad hosts and route around failures to maintain throughput.

cc @michaelhood