MB-11843 Use cached seqnos if get_seqs() fail due to connection close

During a node is rebalanced out, the ns_server takes out the partitions
to another node and finally performs a bucket delete. When a bucket is
deleted, EP-Engine closes all dcp connections. During the connection
close occured, if the view_group is requesting for seqnos, it will
fail with dcp_conn_closed error. Retrying to request seqnos after trying
to reconnect socket may not succeed since the bucket is deleted. We
should handle this error gracefully by returning previously cached
seqnos.

Change-Id: Idc24061369f1c29269e482884a3dad169c6a0403
Reviewed-on: http://review.couchbase.org/40404
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>