nsqd: refactor NSQD storage backend format

[andyxning]

As for now, NSQD's backend storage format is not feasible for storing more info and make changes without backward compatibility. We need a rethink and redesign about the serialization format for NSQD's backend storage.

FYI:

• nsqd: keep defer property under nsqd restart #1137 (comment)
• nsqd: keep defer property under nsqd restart #1137 (comment)

[andyxning]

@mreiferson @ploxiln Any of you are working on this. I think i can spare one week to accomplish this.

[ploxiln]

Honestly, I did not get far before pausing in order to work on other projects, and have not yet resumed work on this, some 5 months later. Sorry about that.

[andyxning]

OK, i will prepare a PR to do this refactor. ETA is about one week.

[andyxning]

I think we need to do this, ASAP.