feat(kafka): Start ingester flush loop before trying to catch up from Kafka

[benclive]

What this PR does / why we need it:

Starts the flush loop before starting the kafka catch up logic.

• This avoids a scenario where we are catching up after a long outage/backlog and we aren't flushing chunks, which leads to OOM after a while.
• With the flush loop running, the ingester will be flushing new data as chunks fill up and we shouldn't OOM (as quickly!)

Which issue(s) this PR fixes:
Fixes https://github.com/grafana/loki-private/issues/1149

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR