nsq_to_http: ability to set max backoff time

when using nsq_to_http with `--mode=hostpool` which was added in #174 you may want to blast through messages without backoff allowing hostpool to handle routing around failures, and allowing nsq to do per-message backoff/retry.

To do this, Reader should be improved to skip backoff if the MaxBackoffDuration is 0, and nsq_to_http should expose a flag to set the max backoff duration.