Inconsistent handling of *Handlers

[fbricon]

Some *Handler files return CompletableFutures while others return raw types and let the JDTLanguageServer handle the wrapping into CompletableFuture.
eg.

	@Override
	public CompletableFuture<String> classFileContents(TextDocumentIdentifier param) {
		logInfo(">> java/classFileContents");
		ClassfileContentHandler handler = new ClassfileContentHandler();
		return  handler.contents(param);
	}

vs

	@Override
	public CompletableFuture<CodeLens> resolveCodeLens(CodeLens unresolved) {
		logInfo(">> codeLens/resolve");
		CodeLensHandler handler = new CodeLensHandler(preferenceManager);
		return CompletableFuture.supplyAsync(()->handler.resolve(unresolved));
	}

The *Handler behavior should be consistent across the board. I say we leave JDTLanguageServer deal with the CompletableFuture aspects and simplify the Handlers, which will simplify their testing as well.