Improve: Performance tuning

[HerringtonDarkholme]

This issue would be a meta issue, logging all the field we can improve performance.

Async

Currently we return result in sync fashion. However, vscode-langauge-server accepts Promise based return value.

Sync value has several drawbacks:

1. One single slow command blocks the whole server
2. Hard to exploit multiple CPUs
3. No way to use dynamic import, this blocks lazy loading

We can change our underlying service interface to support it.

Component Resolution

We now uses a single findComponents function to resolve all component info. This might make our completion laggy. #436 . But TagProvider, the consumer of findComponents, does require one single component at a time.

We can improve findComponents and related functions to do less work in completion.

Block Doc Version & Incremental Change.

Vetur split different SFC block into different TextDocument. They are share the same version. Thus, updating in template block will force vetur to unnecessarily reparse script block.

[octref]

Not using async actually blocks some features. I'll do the migration soon.

[octref]

This would block #682 so doing it now.