Fix byte compiler errors and Emacs compatibility

[bwolf]

Mainly makes this package usable on Emacs 26.1 when it is byte compiled. The byte compiler signals multiple errors without the change.

[bwolf]

Hm, I will check the CI failures and report here.

[bwolf]

I checked the build output and cannot explain why it failed. Maybe a timing issue. I verified the tests using in a local Docker container based on silex/emacs:25.1-dev and run:

docker run --rm -it -v $PWD:/data silex/emacs:25.3-dev /bin/bash
apt-get update && apt-get install -y openjdk-11-jdk
cd /data
cask install && cask build && cask exec ecukes --timeout 60

I repeated the above steps successfully for silex/emacs:25.2-dev, silex/emacs:25.3-dev and finally silex/emacs:22-dev.

[yyoncho]

@bwolf ignore the failures - one of the instances has succeeded that is enough. The tests are rather unstable and tend to fail from time to time due to timing issues related to the server. I will do the review when I back from work.

[yyoncho]

@bwolf thank you for your hard work and congrats for your first lsp-java PR! Please let me know here or in gitter if you are interested in other lsp/dap related PRs. We probably need to do the same change for lsp-mode at some point.