apps: refactor reader command line options

this removes the individual reader command line flags in favor
of a single --reader-config flag that is expected to be a JSON
string passed directly to a new reader.Configure().

nsq_to_file:
    * --gzip-compression is now --gzip-level
    * dropped --verbose (now set via --reader-config)
    * dropped --max-in-flight (now set via --reader-config)

nsq_to_http:
    * dropped deprecated --round-robin (--mode default is "round-robin")
    * --throttle-fraction is now --sample
    * --http-timeout-ms is now --http-timeout (is now a duration flag)
    * dropped --verbose (now set via --reader-config)
    * dropped --max-in-flight (now set via --reader-config)
    * dropped --max-backoff-duration (now set via --reader-config)

nsq_to_nsq:
    * dropped --verbose (now set via --reader-config)
    * dropped --max-in-flight (now set via --reader-config)
    * dropped --max-backoff-duration (now set via --reader-config)