[vibe-d/mongodb] [important] [mongo8] OP_COMPRESSED defined but not implemented

[gedaiu]

We have the OpCode.Compressed enum value, but never actually compress anything. No snappy, zlib, or zstd, and we don't negotiate compression in the handshake.

Matters most for large documents or high throughput. zstd gives the best compression, and snappy is the cheapest on CPU.