Server implementation should provide document validation queuing

[dbaeumer]

Currently linters are implemented by directly reacting in document changes (they validate when the change comes in). If n new requests come in while the document is active on 2 validation request should be executed. Currently most linters execute n + 1 validation request.

The implementation could make use of vs/base/common/async.ts#Throttler

[dbaeumer]

Started to work on this. Still in beta.

[dbaeumer]

I made the whole json rpc message processing more async. I din't implement a queue to reuse since I couldn't come up with a good folding story. So I decided to leave this up for the server implementors (e.g eslint for example does implement a special queuing taking special eslint needs into consideration)