Add MaxFlushDelay option

[FZambia]

Relates #156

Adds MaxFlushDelay option to rueidis.ClientOption:

// MaxFlushDelay when greater than zero pauses pipeline write loop for some time (not larger than MaxFlushDelay)
// after each flushing of data to the connection. This gives pipeline a chance to collect more commands to send
// to Redis. Adding this delay increases latency, reduces throughput – but in most cases may significantly reduce
// application and Redis CPU utilization due to less executed system calls. By default, Rueidis flushes data to the
// connection without extra delays. Depending on network latency and application-specific conditions the value
// of MaxFlushDelay may vary, sth like 20 microseconds should not affect latency/throughput a lot but still
// produce notable CPU usage reduction under load.
MaxFlushDelay time.Duration