XDCR should not ask for all DCP stats from KV

I've noticed XDCR appears to collect the `"dcp"` stats group from KV. It appears the output is retained in memory.

https://github.com/couchbase/goxdcr/blob/c82c4d94cc4c595b237c12db4d77cf73b42d949d/service_impl/bucketTopologyService.go#L227

This stat group contains detailed stats for every DCP stream (multiple streams per producer) in KV, and can generate a lot of data and stall the connection. We've previously seen timeouts of 60s during cbcollects, and 100s MiB of output (see linked MBs).

The stat group supports a filter, passed in as the value of the request, used to ignore streams not belonging to a specific user, something like `{"user": "@goxdcr"}`. Ns_server has started doing that recently: https://review.couchbase.org/c/ns_server/+/200841/7/src/dcp_replicator.erl#298

Additionally, as of 7.6.0, XDCR can specify "stream_format" to ignore the per-vBucket stream stats, if not required, as they are the bulk of the output.

https://github.com/couchbase/kv_engine/blob/master/docs/BinaryProtocol.md#statdcp

My worry here is that if XDCR asks for that STAT group and there is a very large output to generate, XDCR will observe a very large payload on that connection, and potentially delay other operations in a non-obvious way (theory).