Implement Background Service

[Krzysztof-Cieslak]

Ok, this is huge. It's evolution of current SymbolCache and implements some of the ideas I've described in Krzysztof-Cieslak/notes#1

1. SymbolCache project is removed, and we have new project called BackgroundService.
2. BackgroundService communicates with the FSAC using JSON-RPC (implementation from our LSP implementation)
3. With BackgroundService enabled (hopefully turned on by default in Ionide), FSAC doesn't perform almost any background processing - it's focused only on responding to the requests that happened and typechecking current file. It means that there is much smaller pressure on reactor queue and FSAC is much more responsive. (IT'S FAST)
4. With BackogrundService enabled, FSharpChecker used by FSAC doesn't need to cache background resolutions which decrease memory usage (just in FSAC process)
5. With BackgroundService enabled, FSharpChecker used by FSAC doesn't implicitly start background processing
6. File state is synchronized between FSAC and BackgroundService.
7. BackgroudnService is responsible for typechecking files from current project, getting errors and pushing them to FSAC (that pushed it forward to client)
8. BackgroundService is responsible for finding all symbols and usaged in typechecked file and puts that into SQLite DB
9. Symbols are not longer kept in memory - it turns out SQLite is fast, and we may just query local DB directly from FSAC when needed
10. Background service has also functionality for typechecking whole projects (on file save) but it's disabled due to the CPU and memory usage this creates.
11. As described in the document linked above - it probably increases total memory and CPU usage [EDIT: probably not so much - there is visble decrease in FSAC CPU and memory usage due to not keeping background resolutions, lack of background typechecking and lack of symbol calculations] ... but in a way that's not visible to the user - FSAC is always responsive.