libs/db: conditional compilation (#3628)

* 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

config/config.go

@@ -153,7 +153,18 @@ type BaseConfig struct {
 	// and verifying their commits
 	FastSync bool `mapstructure:"fast_sync"`
 
-	// Database backend: leveldb | memdb | cleveldb
+	// Database backend: goleveldb | cleveldb | boltdb
+	// * goleveldb (github.com/syndtr/goleveldb - most popular implementation)
+	//   - pure go
+	//   - stable
+	// * cleveldb (uses levigo wrapper)
+	//   - fast
+	//   - requires gcc
+	//   - use cleveldb build tag (go build -tags cleveldb)
+	// * boltdb (uses etcd's fork of bolt - github.com/etcd-io/bbolt)
+	//   - EXPERIMENTAL
+	//   - may be faster is some use-cases (random reads - indexer)
+	//   - use boltdb build tag (go build -tags boltdb)
 	DBBackend string `mapstructure:"db_backend"`
 
 	// Database directory
@@ -207,7 +218,7 @@ func DefaultBaseConfig() BaseConfig {
 		ProfListenAddress:  "",
 		FastSync:           true,
 		FilterPeers:        false,
-		DBBackend:          "leveldb",
+		DBBackend:          "goleveldb",
 		DBPath:             "data",
 	}
 }