Sidekiq Memory Killer will always kill a large import

When importing a large project, the git process can spawn and easily use more than 1+ GB RAM, tripping the Sidekiq memory killer:

The workaround is to disable the importer:

gitlab_rails['env'] = {
'SIDEKIQ_MEMORY_KILLER_MAX_RSS' => '0'
}

This isn't a good experience and causes the importer to fail in odd ways if the repository fails to import. Some ideas:

1. We whitelist certain processes (e.g. git) from the total RSS calculation.
2. We extend the grace period (now: 15 minutes) for when Sidekiq actually goes down.