modify version and readme to release 2.0.0 (apache#469)

fix(producer): update name server address immediately when client sta…

…rt ; or end up with a failed publish (apache#436)

[ISSUE apache#354] feat: Support PanicHandler (apache#355)

* feat: Support PanicHandler

Closes apache#354

[ISSUE apache#301] Adding GET_CONSUMER_RUNNING_INFO (apache#311)

* Adding GET_CONSUMER_RUNNING_INFO impl

* add Marshal ConsumerRunningInfo Test Case

fix(transaction):can not do local execute and check (apache#262)

feat(demos): add seperately demos

[ISSUE apache#75] Add release note for 2.0.0-alpha2

fix panic bugs when topic not exist (apache#63)

update Introduction (apache#24)