* libs/db: conditional compilation
For cleveldb: go build -tags cleveldb
For boltdb: go build -tags boltdb
Fixes#3611
* document db_backend param better
* remove deprecated LevelDBBackend
* update changelog
* add missing lines
* add new line
* fix TestRemoteDB
* add a line about boltdb tag
* Revert "remove deprecated LevelDBBackend"
This reverts commit 1aa85453f7.
* make PR non breaking
* change DEPRECATED label format
https://stackoverflow.com/a/36360323/820520