Use latest language features

[chkr1011]

This PR applies several automatic code changes to to support of new language features. Most of them have no impact regarding functionality (i.e. collection initializer, init instead of internal, using). Only a few lead to slightly different behavior (i.e. async dispose).

There a no new features in this PR. Applying these refactoring yet ensures that further feature PRs are not cluttered with these changes.