This sets up all the basics for importing Phabricator tasks into
GitLab issues.
To import all tasks from a Phabricator instance into GitLab, we'll
import all of them into a new project that will have its repository
disabled.
The import is hooked into a regular ProjectImport setup, but similar
to the GitHub parallel importer takes care of all the imports itself.
In this iteration, we're importing each page of tasks in a separate
sidekiq job.
The first thing we do when requesting a new page of tasks is schedule
the next page to be imported. But to avoid deadlocks, we only allow a
single job per worker type to run at the same time.
For now we're only importing basic Issue information, this should be
extended to richer information.
Gitlab::HTTP now resolves the hostname only once, verifies the IP is not
blocked, and then uses the same IP to perform the actual request, while
passing the original hostname in the `Host` header and SSL SNI field.
