ServerClient: Flush cached data

ServerClient: Flush cached data

If daemon is using SASL it reads client data into a cache. This cache is
big (usually 65KB) and can thus contain 2 or more messages. However,
on socket event we can dispatch only one message. So if we read two
messages at once, the second will not be dispatched as the socket event
goes away with filling the cache.
Moreover, when dispatching the cache we need to remember to take care
of client max requests limit.