* 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 1aa85453f76605e0c4d967601bbe26240e668d51.
* make PR non breaking
* change DEPRECATED label format
https://stackoverflow.com/a/36360323/820520
* Implement issues 2386: add synchronization in lite verify and change all Certify to Verify
* Replace make(chan struct{}, 0) with make(chan struct{})
* Parameterize memroy cache size and add concurrent test
* Refactor import order
