Make commands wait for applyEdit

[fvclaus]

I stumbled over this trying to find out why organizeImports in combination with Prettier and formatDocument would always alternate its output. My configuration looked like this:

        "editor.codeActionsOnSave": [ 
            "source.organizeImports",
            "source.formatDocument",
        ]

The list is executed in order and the commands are awaited for completion. I must organize imports before saving to get rid of unused imports and because I cannot recreate Prettier's import order using vscode-java. (More details here: https://gitter.im/redhat-developer/vscode-java?at=60e84814f1274d5e503bf7d8)
Trying to recreate this issue with a simple extension, I found out that this occurs, when the command returns before all its asynchronous work is done.

After my changes, the behavior changes to the following:

I applied the changes to all commands that use applyWorkspaceEdit. I used the code for about a week and did not find any (new) problems.

[testforstephen]

Since I don't know how to use Prettier Java extension in VS Code, I cannot reproduce the conflict issue between organizeImport and prettier.

But regarding this change, i don't see any bad effects of returning a promise in the applyEdit command. So i'd like to merge it.