Coroutinize http client code

[xemul]

The code looks nicer and with much less indentation depth that way

[xemul]

upd:

• removed unused ex variable
• simplified retry coroutinization in client::do_make_request (con-less overload) patch

[gleb-cloudius]

As a general note when I tried to co-routinize cql server in Scylla I did not find a way to do so without introducing more allocations when requests are pipelined. With continuations if we read more than one request from a socket all of them can be processed without allocating continuations since futures will be ready. With co-routins we always allocate.

[avikivity]

With [[clang::coro_elide_safe]] nested coroutines can share the coroutine frame. However it only arrives in clang 20 and isn't standardized.

[nyh]

Hi @xemul,

When you created a 28-patch series which must have taken you many many hours to create, please spend another 10 minutes trying to explain your motivation for doing this. Was it just the generic "coroutines are nicer" sentiment that we all shared, or something specific which bothered you?

Also, I see a lot of discussion regarding the lower performance of the coroutine code, so I wonder if you made an attempt to benchmark the http server somehow to see if there's anything we need to worry about.

[xemul]

Hi @xemul,

When you created a 28-patch series which must have taken you many many hours to create, please spend another 10 minutes trying to explain your motivation for doing this. Was it just the generic "coroutines are nicer" sentiment that we all shared, or something specific which bothered you?

No, nothing specific. It just looks much nicer and simpler this way.

Also, I see a lot of discussion regarding the lower performance of the coroutine code, so I wonder if you made an attempt to benchmark the http server somehow to see if there's anything we need to worry about.

I made a silly test with existing http client demo, but it's not very representative. I'll update the discussion thread with better measurement