Make workspace/symbol with SymbolDescriptor query faster

[felixfbecker]

Symbol search in rxjs:

Possible causes:

• Files searched twice because they belong to both configs. If this is the case, we can first aggregate all SourceFiles to search and dedupe them.
• The many files are compiled per config that don't belong to it
• Getting the package.jsons to check the PackageDescriptor takes a long time
• possibly because the event queue is full and promises always callback async Refactor all async functions to Observables, especially memoized #271
• possibly because other fetches block the Semaphore Replace Semaphore concurrency limit with a prioriy queue #273
  • unlikely, because ensureOwnFiles() is awaited before workspace/symbol so there shouldn't be more fetches queued