config: add rocksdb as a db backend option (#4239)

Closes #3831

The support for rocksdb was added a while back in
https://github.com/tendermint/tm-db/pull/12. This commit merely updates
the documentation.

config/config.go

@@ -160,7 +160,7 @@ type BaseConfig struct { //nolint: maligned
 	// and verifying their commits
 	FastSyncMode bool `mapstructure:"fast_sync"`
 
-	// Database backend: goleveldb | cleveldb | boltdb
+	// Database backend: goleveldb | cleveldb | boltdb | rocksdb
 	// * goleveldb (github.com/syndtr/goleveldb - most popular implementation)
 	//   - pure go
 	//   - stable
@@ -172,6 +172,10 @@ type BaseConfig struct { //nolint: maligned
 	//   - EXPERIMENTAL
 	//   - may be faster is some use-cases (random reads - indexer)
 	//   - use boltdb build tag (go build -tags boltdb)
+	// * rocksdb (uses github.com/tecbot/gorocksdb)
+	//   - EXPERIMENTAL
+	//   - requires gcc
+	//   - use rocksdb build tag (go build -tags rocksdb)
 	DBBackend string `mapstructure:"db_backend"`
 
 	// Database directory

config/toml.go

@@ -86,7 +86,7 @@ moniker = "{{ .BaseConfig.Moniker }}"
 # and verifying their commits
 fast_sync = {{ .BaseConfig.FastSyncMode }}
 
-# Database backend: goleveldb | cleveldb | boltdb
+# Database backend: goleveldb | cleveldb | boltdb | rocksdb
 # * goleveldb (github.com/syndtr/goleveldb - most popular implementation)
 #   - pure go
 #   - stable
@@ -98,6 +98,10 @@ fast_sync = {{ .BaseConfig.FastSyncMode }}
 #   - EXPERIMENTAL
 #   - may be faster is some use-cases (random reads - indexer)
 #   - use boltdb build tag (go build -tags boltdb)
+# * rocksdb (uses github.com/tecbot/gorocksdb)
+#   - EXPERIMENTAL
+#   - requires gcc
+#   - use rocksdb build tag (go build -tags rocksdb)
 db_backend = "{{ .BaseConfig.DBBackend }}"
 
 # Database directory