Hide channels hashmap methods

[SimonWoolf]

A customer had been calling channels.clear() in an on-auth-updated listener, for reasons that remain unclear. Predictably this caused problems.

Might be a good idea to hide the actual hashmap away in a private member (instead of Channels extending HashMap), and remove the temptation of the forbidden fruit of HashMap methods being dangled in front of the user by their IDE's autocomplete

┆Issue is synchronized with this Jira Task by Unito