CouchRocks: Pass the 'purgeSeqno' down to RocksDBKVStore when creating a ScanContext

    return new ScanContext(cb,

                           cl,

                           vbid,

                           scanId,

                           startSeqno,

                           endSeqno,

                           0, /*TODO RDB: pass the real purge-seqno*/

                           options,

                           valOptions,

                           /* documentCount */ endSeqno - startSeqno + 1,

                           configuration);