Stats: Add ChannelConfig & Return error on subscription

[Vigilans]

Add ChannelConfig for app/stats.Channel

A new protobuf message ChannelConfig is added to configure the behavior of stats.Channel:

message ChannelConfig {
    int32 SubscriberLimit = 1;
    int32 BufferSize = 2;
    int32 BroadcastTimeout = 3;
}

With this config, a NewChannel function is added:

func NewChannel(config *ChannelConfig) *Channel {
	return &Channel{
		channel:           make(chan interface{}, config.BufferSize),
		subscriberLimit:   int(config.SubscriberLimit),
		channelBufferSize: int(config.BufferSize),
		broadcastTimeout:  time.Duration(config.BroadcastTimeout+1) * time.Millisecond,
	}
}

So a channel could now be created independently from stats.Manager. All related tests only relying on stats.Channel now removes the depedency of stats.Manager.

Subscribe and Unsubscribe returns error for features/stats.Channel

Since SubscriberLimits in ChannelConfig controls the maximum count of subscribers the Channel could accept at the same time, Channel.Subscribe() does not necessarily succeed and may fail with this config.

So, an error result is returned from Subscribe() and Unsubscribe(ch) now:

interface Channel {
    Subscribe() (chan interface{}, error)
    Unsubscribe(chan interface{}) error
}

[Vigilans]

With Subscribe and Unsubscribe returning error, a new pair of function combining Start/Subscribe and Close/Unsubscribe are added for features/stats.Channel:

• SubscribeRunnableChannel subscribes the channel and starts it if there is first subscriber coming.
• UnsubscribeClosableChannel unsubcribes the channel and close it if there is no more subscriber.

[xiaokangwang]

This PR may be merged. Since the author have modified these file recently and this is an improvement of last PR, I believe merge may be done quickly.