Cleanup in socket/asio code

[randombit]

          Nit: This could now be calld `dns_result`, `endpoints` or something similar, as it is not technically an iterator anymore.

Originally posted by @reneme in #4484 (comment)

Also could use quite a bit of auto love to hide the various contorted type names.

[reneme]

On that note we should probably update this guidance on using auto somewhat:

botan/doc/dev_ref/contributing.rst

Lines 218 to 220 in cc835ca

	The C++11 ``auto`` keyword is very convenient but only use it when the type
	truly is obvious (considering also the potential for unexpected integer
	conversions and the like, such as an apparent uint8_t being promoted to an int).

... probably better to extend to something like "... truly obvious or not particularly relevant to reason about the semantics".

[randombit]

truly obvious or not particularly relevant to reason about the semantics

👍