Make sure we stop the current client before restarting

[vinistock]

Closes Shopify/ruby-lsp#1505

Given this scenario:

1. User starts the Ruby LSP on a project that needs bundle install
2. Instead of clicking on bundle install, run bundle install through the terminal
3. Change Gemfile, Gemfile.lock or any other watched files
4. This would trigger a restart and spawn a server process (which would work because gems are now installed)
5. After this process has been spawned, click bundle install on the dialogue
6. That would spawn a second LSP client and two processes would run

The reason this happens is because

1. We weren't awaiting the call to restart
2. While the dialogue was waiting for an answer, we hadn't set this.client. That means that invoking stop didn't create a stop promise, because the client wasn't set yet

The fix is to properly await the promises and set this.client early, so that we can properly return a stop promise when restarting a client that is hanging waiting for a dialogue confirmation.