nsq_to_nsq: default channel name should be "nsq_to_nsq"
nsq_to_nsq: add basic message filtering options

Specify --require-json-field to only pass (json) messages that have that field, or
also specify --require-json-value to only pass (json) messages in which the
string or number value of that field matches.

Specify any number of --whitelist-json-field to only pass those fields in
the json messages being relayed, stripping all other fields.

These options only work with messages that are json objects, and it can
only work with top-level members of those objects (but it can pass
through arbitrary child structures).

If you don't use these options, nsq_to_nsq still works with arbitrary messages.

also use common getRequeueDelay() function everywhere