DriverStatusHTTPHandler causes timeout to Http Request if there is no Driver status change

As a HTTP Handler, it should response to every HTTP request to the endpoint "driverstatus/v1". 

However, instead of recording the current Driver status, in DriverStatusHTTPHandler, it uses a queue to only save the change. If there is no change in Driver status, every HTTP requests will timeout. 

The problem is in:

https://github.com/apache/reef/blob/master/lang/java/reef-bridge-client/src/main/java/org/apache/reef/bridge/client/DriverStatusHTTPHandler.java#L154

 Reproduce:

1. Modify HelloTask - add a while loop to print "Hello, REEF!" to prevent driver from exiting.
2. Send HTTP request "http://IP:Port/driverstatus/v1" to query driver status multiple times.

Observation:

1. I am able to see Driver Status "INIT", "RUNNING"
2. From this point on, all HTTP requests to this handler will timeout, unless Driver status changes again.

Expectation:

HTTP server should response with current Driver status at anytime.