Sets the name for the stored WAL segment to be a ULID with a prefix used only by this ingester.
This will need more work as the entropy source is not concurrent safe & will probably collide with another instance of the ingester, but I wanted to namespace the object path before trying this code against a real object store to avoid polluting it.
What this PR does / why we need it:
Which issue(s) this PR fixes: Partial implements https://github.com/grafana/loki-private/issues/1020