Async support to gitlab-http

After the initial implementation of the gitlab-http gem (#415686 (closed)), we'll add the async support to it in this issue.

This will unblock #351250 (closed).

Implementation table

1. Add async support to gitlab-http (!135127 - merged)
2. Prevent gitlab-http async to use DB and Gitaly (!136294 - merged)
3. Clear Sentry and Raven settings after the relat... (!138304 - merged)
4. Remove restrict_within_concurrent_ruby after ch... (!138323 - merged)