Eager load http-cookie to avoid thread-safety bug

Without this sometimes we can see this error while running for the
first few requests:

    ArgumentError:
      cookie store unavailable: :hash

That's because http-cookie is not loading the store in a thread-safe
way.

See: https://gitlab.com/gitlab-org/gitlab/issues/207937

config/initializers/0_eager_load_http_cookie.rb

+# frozen_string_literal: true
+
+# https://gitlab.com/gitlab-org/gitlab/issues/207937
+# http-cookie is not thread-safe while loading it the first time, see:
+# https://github.com/sparklemotion/http-cookie/issues/6#issuecomment-543570876
+# If we're using it, we should eagerly load it.
+# For now, we have an implicit dependency on it via:
+# * http
+# * rest-client
+require 'http/cookie_jar/hash_store' if Gem.loaded_specs.key?('http-cookie')