merge volume metadata store into meta module

[rudyfly]

Issue Description
volume metadata store is different from container store, we should merge them into one module.

[allencloud]

Friendly ping, @rudyfly
And can you sync the progress a little bit with us? 🥂

[skoowoo]

Will You let the metastore to support multi backend driver ? If so, I think it's okay. @rudyfly

[allencloud]

Since this is a 20-days issue, could you sync with us the latest status of that? @rudyfly

[rudyfly]

I haven't started this job, can I pause this job?

[allencloud]

Thanks for feedback. Actually in my opinion, you can leave this issue just there. However maybe if you are current not on this part, you could sync with others on github under this issue, then more people know that this work may not be in your working queue currently. And it will improve a little bit in project management. @rudyfly