Multiple DefaultActionDispatcher are created and not properly disposed

[martin-fleck-at]

Steps to reproduce:

• Start the Java server and the standalone example
• There is a temporary client session for each diagramType created to retrieve the diagram type specific
• Check that there are two action dispatcher threads created
• Reload the page
• See that one action dispatcher thread remains and two new are created
• Continue to see the number of remaining action dispatcher threads rising

It seems to me that the problem is that the action dispatcher also serves as an action handler for the internal JoinAction. The multi-binding does not ensure that we re-use the existing action dispatcher which would be achievable through a provider binding or other methods.

[ivy-lli]

PR: eclipse-glsp/glsp-server#246