PIP for making dispatcher pluggable

[Zhengxin Cai <ca...@gmail.com>]

Hi Pulsar community,

Recently there was discussion about adding consumer filtering logic at
broker side, and the community has choose not to add that to core
components but still would like to find a way for uses to do their own
customization.
Here I'm proposing we can make the message dispatcher pluggable and add
factory for creating all kinds fo dispatchers. Users can create their own
dispatchers and factory and can use customized dispatcher with business
logic like filtering there.
A more detailed proposal in this google doc:
https://docs.google.com/document/d/1RxgK833SgbEWvso-9xJuSsT4he5Yqw3E0AIGp3GmoNI
Any comments are welcome.

Best,
Marvin,