MB-12036: Handle new stats when set up dcp streams

Besides existing high_seqno and uuid, dcp introduces new stats
such as abs_high_seqno and purge_seqno. We need to abstract these
stats correctly even if we dont consume them for the time being.

Change-Id: I6a53f1b2b96ddffd7f147b72c368bcee77fe1a74
Reviewed-on: http://review.couchbase.org/40767
Tested-by: Bin Cui <bin.cui@gmail.com>
Reviewed-by: Steve Yen <steve.yen@gmail.com>