Next Major: Make classes final where possible

[dbu]

Q	A
Bug?	no
New Feature?	no
Version	BC breaking change: Next major

Classes like the factories are not final. We should go over the bundle and make everything final that is not explicitly intended to be extended. Also check for things with protected visibility and make them private where possible.

[xabbuh]

For one of the next 1.x releases we can already flag these classes with the @final annotation in the class docblock. The DebugClassLoader from the Symfony Debug component will then raise a deprecation if someone extends these final classes without a hard break.

[andrew-demb]

Since there's no final decision from the @php-http/owners on the discussion #321 (comment) - should this issue be considered to be a blocker for 2.0.0?

@dbu

[xabbuh]

fixed in #467