Increase the schedule frequency of HeartbeatTask.

[chickenlj]

Here is how heartbeat works in Dubbo:
Heartbeat scheduler will schedule HeartbeatTask in every heartbeat time, when being scheduled, HeartbeatTask will internally continue to check the necessity of sending a heartbeat event (only send when it find that the idle interval >heartbeat).
Consider different combinations of these two intervals: schedule interval and idle interval, we may have a chance of idle for 2*heartbeat on a wire without any request/response or heartbeat event being sent.

Though the timeout interval in Dubbo is 3*heartbeat, and 2*heartbeat idle would not mislead Dubbo to terminate the connection, we may need to increase the frequency of the scheduler to make it more accurate.

[chickenlj]

https://github.com/apache/incubator-dubbo/blob/cd60e58e1a579749c89c6ac8ddb39f8e8e894ef5/dubbo-remoting/dubbo-remoting-api/src/main/java/org/apache/dubbo/remoting/exchange/support/header/HeaderExchangeClient.java#L186

[carryxyh]

Hi, I will start a work on it.
I plan to use hashwheeltimer to make each client's heartbeat more frequent.
What's your opinion?

[carryxyh]

Close via #2658.