Provide Progress Notification

[tsmaeder]

jdt.ls does a bunch of long running tasks (build, update maven project, indexing jars, etc.) that can result in high system load and affect outcomes of stuff like content assist.
These tasks may be done in response to a LSP request or notification or they may be triggered by jdt.ls itself.
We should have a way to report progress to the client. Some of the issues that need to be addressed are:

• Multiple tasks may be active in parallel, so we need a way to identify the tasks
• We must not send too many notifications
  Progress reporting is for humans: more than 2-3 notifications/second cannot be seen anyway.

[gorkem]

See also microsoft/vscode-languageserver-node#261

[tsmaeder]

There is a WIP PR here: #490