Add monitor feature

[kukey]

implents #652

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✅ kukey
❌ huangwei03

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[TysonAndre]

Seems promising and should have low impact when monitors are not used, but I had some review comments.

I'm also not that familiar with the redis monitor protocol and the edge cases, so I'll probably have more comments when I do look at that.

E.g. twemproxy should probably disconnect a client and assume the client is misbehaving if a command is sent by a client (with c->monitor true) before the client signals that the monitor should be stopped.

[kukey]

@TysonAndre
I suddenly realized the need to add a security authentication mechanism to prevent disclosure of request information caused by monitor command.

[TysonAndre]

I suddenly realized the need to add a security authentication mechanism to prevent disclosure of request information caused by monitor command.

Also, it would be reasonable to have it be off by default in the pool config (nc_conf) - if nobody's actively using it or deciding to enable it the performance impact would be surprising when someone tried monitor

[kukey]

@TysonAndre please review

[kukey]

i review again, please give me some suggestion, thanks. @TysonAndre