sql: Add migration for mxq_daemon.tags

We want to rename prerequisites to tags. Additionally, we want a new
column prerequisites with new data. However, we can't go the direct
way (CHANGE COLUMN prerequisites tags...), because the running mxqd
processes use the old name.

Create a new column with the new name and copy the data over. We don't
really care to much, if data is lost because a mxq daemon restarts
during the migration phase, because the former prerequisites and now tags
column is only published into the database.

Additionally, fix length of prerequisites from 1024 to 1000.

mysql/migrate_010_add_mxq_daemon_tags.sql

@@ -0,0 +1,9 @@
+ALTER TABLE mxq_daemon MODIFY COLUMN prerequisites VARCHAR(1000) NOT NULL DEFAULT '';
+
+ALTER TABLE mxq_daemon
+  ADD COLUMN
+    tags VARCHAR(1000) NOT NULL DEFAULT ''
+  AFTER
+    prerequisites;
+
+UPDATE mxq_daemon SET tags=prerequisites;