update to Badger DB 3.0

[chappjc]

https://github.com/dgraph-io/badger#project-status-march-24-2020

v1 -> v2 is breaking

Try it out, adjust our code, and evaluate performance.

Also solve rebuild of pool DB without complete sync of entire dcrdata DB.

[ukane-philemon]

@chappjc Badger v3 has been released with lots of performance fix and changes.
v2 - v3 is breaking.
v1 - v3 is better?

[chappjc]

AFAICT, v2 is not actively developed anymore.
The first v3 release was over a year ago now, so it is probably stable. However we have to be careful not to require cgo, which I think takes special build options with v3. Please do look into it. https://github.com/dgraph-io/badger/blob/master/docs/content/faq/index.md#why-do-i-need-gcc-to-build-badger-does-badger-need-cgo
EDIT: oh actually they removed the zstd cgo thing: https://github.com/dgraph-io/badger/releases/tag/v3.2103.1 That's good.