Merge pull request #2 from tiny657/master

support partial callbackHandler.
Merge pull request #3 from tiny657/master

fixed handler & detect the the lost broker info and the unsent data.
notify the unsent data to the handler when the producer is closed.
return the size of the remained blocking queue for monitoring
Merge pull request #5 from tiny657/handler

notify the unsent data to the handler when the producer is closed.
Merge pull request #6 from tiny657/queueSize

return the size of the remained blocking queue for monitoring