JNI ColumnFamilyHandle#getDescriptor method leaks memory

### Expected behavior
Calling `ColumnFamilyHandle#getDescriptor()` should produce either a `ColumnFamilyDescriptor` that can be `close()`d to free the native memory backing it, or no native allocation should occur.

### Actual behavior
Calling `ColumnFamilyHandle#getDescriptor()` allocates a native `rocksdb::db::ColumnFamilyDescriptor`, copying the name and options from the handle. Since the Java `ColumnFamilyDescriptor` is not a `RocksObject`, this memory cannot be tracked/freed from Java.

### Steps to reproduce the behavior
Call `ColumnFamilyHandle#getDescriptor()` repeatedly from Java and monitor memory usage.