Add heartbeat to import jobs to avoid timeout if the job is still working

From https://gitlab.com/gitlab-org/gitlab-ce/issues/38771#note_42521952

Right now mirror/fork/import jobs that have been running for more than 15 hours are marked as stuck due to time out

• This is not always the case since a import job can potentially take more than that to process

• @DouweM proposed that we could send, inside the worker, a heartbeat periodically os that the worker does not timeout while we are processing information.