Ephemeral: Handle DCP Cursor Dropping

In Couchbase buckets, high memory usage due to DCP clients is handled by using "cursor dropping". That is, in memory cursors are dropped and we fall back to disk backfills. However, in Ephemeral we have in-memory backfills instead and they further add to memory usage.

We must handle cursor dropping either by i) dropping the stream ii) falling back to backfills and then memory managing backfills.

(Backfill memory management ticket is related to this)