Bring RocksDB to PostgreSQL as an extension. It is the first foreign data wrapper (FDW) that introduces LSM-tree into PostgreSQL. The underneath storage engine can be RocksDB. The FDW also serves for VidarDB engine, a versatile storage engine for various workloads. See the link for more info about VidarDB engine.
PostgreSQL can add additional process then manage it. I think we should let PostgreSQL manage the process spawned by us.