apps/nsq_to_file: supported rotate by size & interval, added <PID> format tag 


apps/nsq_to_file: supported rotate by size & interval
    
    ADD:
    --rotate-size=0,        rotate the file when it grow bigger than `rotate-size` bytes
    --rotate-interval=0,    rotate the file every `rotate-interval` seconds
    --max-revision=1000,    the max <REV> number
    
    CHANGE:
    1. rename <GZIPREV> to <REV> in filename format
    2. reduce `Ticker` duration from 30s to 10s.
    3. rotation only triggered by `Ticker`
    4. `Sync` before `Close`
    
    REMOVE:
    --skip-empty-files,     deprecated, no longer used.

apps/nsq_to_file: add <PID> filename format tag

right, I had mistakenly assumed that we wanted to count input bytes but I agree that doesn't make sense - I think your previous suggestion should work.