Add purge_seq to DbInfo

Change-Id: I4dd9c337dfedf3e3949f1dcedfaa87462f9b10b8
Reviewed-on: http://review.couchbase.org/35062
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>