Fix empty waiting_queue warning

[doyoubi]

The Problem

There are lots of "server is readable but waiting_queue is empty" warnings in our logs when connections are closed by redis. This log was mainly used to find whether this case exists: we didn't send any command to redis but still receive response data. However, now this warning arose just because we didn't check whether the readable event is to close connection.
Most of the time this will not do harm. But in a rare case this will result in "-ERR Proxy error\r\n" response. Here's how it happen:
(1) Client send command to corvus
(2) corvus save this command in ready_queue of the connection between corvus and redis and wait for the writable event to forward this command to redis.
(3) Before corvus send the command, redis close the connection because of timeout.
(4) corvus receive readable event from redis but find its waiting_queue is empty. Then it send the Proxy error response to client.

How To Solve It

Distinguishing whether the readable event is to close connection is the perfect solution, but this require changing many codes so I think it's not worth it.
An easy workaround is to let corvus always close the connection before redis.
Firstly, use a server_timeout in corvus config which is smaller than the timeout in redis config, for example 500 for server_timeout and 600 for timeout. The gap between these two timeouts should be big enough to prevent the case that when corvus is about to send request to redis, redis close the connection at the same time.
Secondly, instead of the timeout of command request to redis, change server_timeout to be the connection timeout. This is exactly what this PR do.