RocksDB Java API for reopening a database

RocksDB Java API is difficult to use in combination with Spring. In Spring, beans are created once and "wired" everywhere they are needed at application startup. It's all but impossible to re-wire Spring beans, short of restarting the application. This makes the `RocksDB.open()` API, which always returns a new object, difficult to use when a database needs to be re-opened. A typical use-case when this is required is a web server, which opens a database read-only, and only during maintenance needs to re-open it for writing.

Thus hereby a request to implement a `reopen()` method for reopening a closed `RocksDB` instance.