code: function queue (job, value, put) {
        var ts = timestamp()
        var key = toKey(job, ts)
  
        var id = hash(job+':'+value)
        if(pending[id]) return null //this job is already queued.
        pending[id] = Date.now()
  
        if(put === false) {
          //return the job to be queued, to include it in a batch insert.
            return {
            type: 'put', 
            key: Buffer.isBuffer(key) ? key : new Buffer(key), 
            value: Buffer.isBuffer(key) ? value : new Buffer(value)
          }
        } else {
          db.put(key, value)
        }
      }
docs: put=false means return a job to be queued