Refactor Engine/DB Key/KeyID handling

Question

• Separate database key IDs from engine keys to reduce database queries while holding the engine key table lock
• Add support for directly accessing the byte array of BinaryEncoding

该提问来源于开源项目：apple/swift-llbuild

Answer 1

In the synthetic llvm ninja build benchmark this is not a clear win. Indeed, it seems to make the test more variable, rather than always good or always bad.