Set id as the PRIMARY KEY of AttachmentsIndex for Pg

This is consistent with mysql's fulltext table. As id is the primary
key, DB automatically indexes the column which helps performance
especially for the following SQL that is called quite frequently when
indexing data:

    SELECT MAX(id) FROM AttachmentsIndex

etc/upgrade/5.0.6/schema.Pg

@@ -0,0 +1,2 @@
+ALTER TABLE IF EXISTS AttachmentsIndex DROP CONSTRAINT IF EXISTS attachmentsindex_pkey;
+ALTER TABLE IF EXISTS AttachmentsIndex ADD PRIMARY KEY(id);

sbin/rt-setup-fulltext-index.in

@@ -327,7 +327,8 @@ elsif ( $DB{'type'} eq 'Pg' ) {
         push @schema, split /;\n+/, <<SCHEMA;
 CREATE TABLE $table (
     id BIGSERIAL,
-    $column tsvector
+    $column tsvector,
+    PRIMARY KEY (id)
 );
 GRANT SELECT, INSERT, UPDATE, DELETE ON $table TO "$DB{user}"
 SCHEMA