if fast-completion is enabled. So they have higher chance to use the cached
completion instance.
If it's disabled, don't block, use an ephemeral instance so we can peform
multiple completions simultaneously.
To controls the lifetime of CompilerInstance within CompletionIntance.
- Prevent from using the same CompilerInstance from multiple completion
requests
- Separate the stacktrace between "fast" and "normal" completions
- Further code consolidation between various completion-like requests
