Legacy DCP stream from an ephemerial bucket can see snapshot end > highest committed mutation when backfilling

This MB is spinned out from MB-47437 but just to fix the bug for ephemerial buckets.

An explanation of the issue can be found here. However, for ephemerial we'll need to either start recording the high commited seqno per collection as we curren't don't have access to this in memory or perform a range scan to find the high commit seqno for the default collection, before sending the snapshot marker. Given this bug is an edge case when a legacy DCP client connects to a 7.0.X cluster which is using collections it might be worth taking the performance hit.