vendor: update leveldb to 211f780 (poolfix) (#15988)

vendor/github.com/syndtr/goleveldb/README.md

@@ -10,13 +10,15 @@ Installation
 Requirements
 -----------
 
-* Need at least `go1.4` or newer.
+* Need at least `go1.5` or newer.
 
 Usage
 -----------
 
 Create or open a database:
 ```go
+// The returned DB instance is safe for concurrent use. Which mean that all
+// DB's methods may be called concurrently from multiple goroutine.
 db, err := leveldb.OpenFile("path/to/db", nil)
 ...
 defer db.Close()