added composite index for seq_member_id and gene_member_id to seq_member

We have some very large (>10000) bacterial gene families, and retrieving the members one by one is quite slow. If we use direct SQL to retrieve some details by joining family_member to seq_member to gene_member, the operation is made much faster (20s vs 0.3s) by adding a composite key of seq_member_id and gene_member_id to the seq_member table.