remove dependency for github.com/dgraph-io/ristretto/z

[mattn]

dgraph-io/ristretto use google/glog. glog rewrites flags on its own, which causes unexpected behaviour.

For example:

package main

import (
	"flag"
	//_ "github.com/dgraph-io/ristretto/z"
)

func main() {
	flag.Parse()
}

depend on dgraph-io/ristretto

$ ./test -h
Usage of ./test:
  -alsologtostderr
    	log to standard error as well as files
  -log_backtrace_at value
    	when logging hits line file:N, emit a stack trace
  -log_dir string
    	If non-empty, write log files in this directory
  -logtostderr
    	log to standard error instead of files
  -stderrthreshold value
    	logs at or above this threshold go to stderr
  -v value
    	log level for V logs
  -vmodule value
    	comma-separated list of pattern=N settings for file-filtered logging

not depend on dgraph-io/ristretto

$ ./test -h
Usage of ./zz:

[sweep-ai]

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• Apply: All new business logic should have corresponding unit tests.
• Apply: Refactor large functions to be more modular.
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