Support canceling a debug request from the client

[akaroml]

https://code.visualstudio.com/updates/v1_39#_cancellation-support

In the 1.39, vscode added cancellation support in DAP. It tells the server that the client is no longer interested in the response of a previous request. This is potentially useful for time-consuming requests like evaluation, stacktrace, scopes, and variables.

Let's investigate whether this is possible with JVM.