Make indexing more evented

[felixfbecker]

Currently initialize blocks sending a response until indexing finished.
It should return instantly and kick off indexing in the background.
Requests that return a single result like hover or j2d should return a result if it is available, otherwise subscribe to new additions to the index and retry. It should also subscribe to indexing being finished, and then return an empty result.
Other methods that return multiple results like references or symbols should defer their response until indexing finished. In the future they might stream their result.