Metadata purge interval reduction for ephemeral buckets

Unlike Couchbase buckets ephemeral buckets store all tombstones in memory. Our default purge interval on a bucket is 3 days, considering the nature of ephemeral buckets these could quickly add-up over days.

We need to reduce the purge interval for ephemeral buckets to a reasonable acceptable size.

Phase I: Field Guidance, guidance to customer via docs and field guidance on the settings

Phase II: Defaults for ephemeral buckets in UI for purge interval (5.1 ??)

Need input from the XDCR team John Liang Yu Sui on what would be an acceptable range for the purge interval taking XDCR into consideration