Make AbstractCurlBackend async friendly

[natsukagami]

Just some structural changes to make implementing an async Curl backend (e.g. with curl_multi_*) possible as
a subclass of AbstractCurlBackend.
Mainly:

• Make header lists per-request.
• Allow a customizable CurlHandle -> F[CurlCode] method, performCurl. This can be done with curl_multi_*
  instead of just curl_easy_perform. Also seems to be the only thing needed to be changed.

We might have to change curl.option calls as well, if we want to properly handle them as monadic calls.

Before submitting pull request:

• Check if the project compiles by running sbt compile
• Verify docs compilation by running sbt compileDocs
• Check if tests pass by running sbt test
• Format code by running sbt scalafmt

[adamw]

instead of vars, mabye we can prepare the headers/multiPartHeaders parameters in send, and then create an (immutable) instance of Context? E.g. case class Context(zone: Zone, headers: Option[CurlList], multiPartHeaders: Seq[CurlList])

[natsukagami]

A bit tricky to do, since setRequestBody is setting up the multi part headers as it goes through the multipart body. I can on the other hand just exposes a addHeader function to create a header that gets cleared when the context is closed.

[adamw]

Thanks for the PR! :) I left some comments/questions inline

[adamw]

(sorry wrong button ;) )

[adamw]

nice catch! :)

[adamw]

Thank you! Do you need a release with these changes, or will you continue working on an async curl backend?

[natsukagami]

Thanks!

Do you need a release with these changes

Not yet, as I am waiting for scala-native 0.5 and everything else to fall into place ;)

will you continue working on an async curl backend

I have a prototype over at https://github.com/natsukagami/sttp/tree/curl-async-gears , but as the branch name suggests, it currently depends on gears stuff and serves mostly as a demonstration that it's possible. I might come back and write a proper backend later :D

[adamw]

I see, thanks for the info, I'll be on the lookout for more PRs then :)