wp_signups table does not have a primary key

[econnell]

There was an old bug from 7 years ago on the WPMU Trac that was marked as "wontfix" for this, however, this is the only table in the WP database schema that is missing a primary key.

Using Percona XtraDB Cluster, a primary key is needed on all tables in order to perform DELETE statements across the cluster as described here: ​http://www.percona.com/doc/percona-xtradb-cluster/limitation.html. It can just be an auto increment "id" column if duplicate entries are needed in this table.

[nacin]

Let's mark this as a duplicate of #15004 so we can go about setting a primary key and an index in one shot.