Fixes#1698.
* Added sync.Pool to take care of many gzip.Writer
allocating a lot of large objects on the heap.
* Reshuffled signal handling, SIGQUIT is now
graceful shutdown, just like in Nginx.
* Signal USR1 prints hepful diagnostic info to stderr.
* Removed gops endpoint and flags.
* Fixed logs in some places.
* Debug flag now adds extra pprof handlers to diagnostic
endpoint.
