avoid infinite recursion by proxying to iterator

```
WARNING: staticcheck, gosimple and unused are all set, using megacheck instead
db/go_level_db.go:126:15⚠️ infinite recursive call (SA5007) (megacheck)
db/go_level_db.go:135:17⚠️ infinite recursive call (SA5007) (megacheck)
db/mem_db.go:106:2⚠️ redundant return statement (S1023) (megacheck)
```

https://circleci.com/gh/tendermint/tmlibs/220

Also remove unnecessary return statement

db/go_level_db.go

@@ -118,12 +118,12 @@ func (db *GoLevelDB) Stats() map[string]string {
 }
 
 type goLevelDBIterator struct {
-	iterator.Iterator
+	source iterator.Iterator
 }
 
 // Key returns a copy of the current key.
 func (it *goLevelDBIterator) Key() []byte {
-	key := it.Key()
+	key := it.source.Key()
 	k := make([]byte, len(key))
 	copy(k, key)
 
@@ -132,13 +132,25 @@ func (it *goLevelDBIterator) Key() []byte {
 
 // Value returns a copy of the current value.
 func (it *goLevelDBIterator) Value() []byte {
-	val := it.Value()
+	val := it.source.Value()
 	v := make([]byte, len(val))
 	copy(v, val)
 
 	return v
 }
 
+func (it *goLevelDBIterator) Error() error {
+	return it.source.Error()
+}
+
+func (it *goLevelDBIterator) Next() bool {
+	return it.source.Next()
+}
+
+func (it *goLevelDBIterator) Release() {
+	it.source.Release()
+}
+
 func (db *GoLevelDB) Iterator() Iterator {
 	return &goLevelDBIterator{db.db.NewIterator(nil, nil)}
 }

db/mem_db.go

@@ -103,7 +103,6 @@ func (it *memDBIterator) Value() []byte {
 func (it *memDBIterator) Release() {
 	it.db = nil
 	it.keys = nil
-	return
 }
 
 func (it *memDBIterator) Error() error {