add some log
try to do more robust UnsavedMessageQueue, consider dataStore write
speed not catch up too often situation
add log to BaseQueryMessageServlet
add BaseListener.pollingType,and create by polling type method
if flush is triggered by resetContinuation, store result in
continuation,makes logic the same as triggered by message push
UnsavedMessageQueue register new listener can controll if history
messages are pushed
fix batch save in UnsavedMessageQueue not check finish bug
add some comment to BaseListener
more strict poll servlet test,check history messages should be sent when
first polling
add more logger to BaseListener
refactor PollServlet