[refactoring] migrate from convention to syntax for private fields

[ngarbezza]

Problem
We're handling private fields using the _field convention. As this is a convention, the fields are still accessible.

Proposed solution
Now we can use #field syntax: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Working_With_Private_Class_Features, that is supported since Node 6, so there are no issues with introducing the syntax to this project.

The proposal is to change it across the whole codebase. We can make chunks of changes in different PRs if that's helpful.

[ngarbezza]

Blocked until this feature enters the official ECMAScript syntax