ethdb: copy stored memdb values

Storing a value in LevelDB copies the bytes, modifying the value
afterwards does not affect the content of the database. This commit
ensures that MemDatabase satisfies the same property.

ethdb/memory_database.go

@@ -36,8 +36,7 @@ func NewMemDatabase() (*MemDatabase, error) {
 }
 
 func (db *MemDatabase) Put(key []byte, value []byte) error {
-	db.db[string(key)] = value
+	db.db[string(key)] = common.CopyBytes(value)
-
 	return nil
 }