Fix fts_unblock_primary test to use content-id instead of dbid.

src/test/isolation2/expected/segwalrep/fts_unblock_primary.out

@@ -24,8 +24,7 @@ content|role|preferred_role|mode|status
 
 -- synchronous_standby_names should be set to '*' by default on primary 2, since
 -- we have a working/sync'd mirror
--- XXX We assume primary 2 is DBID 4, which may not be the case.
-4U: show synchronous_standby_names;
+2U: show synchronous_standby_names;
 synchronous_standby_names
 -------------------------
 *                        
@@ -97,7 +96,7 @@ content|role|preferred_role|mode|status
 COMMIT
 
 -- synchronous_standby_names should now be empty on the primary
-4U: show synchronous_standby_names;
+2U: show synchronous_standby_names;
 synchronous_standby_names
 -------------------------
                          
@@ -127,7 +126,7 @@ insert into fts_unblock_primary select i from generate_series(1,10)i;
 INSERT 10
 
 -- synchronous_standby_names should be back to its original value on the primary
-4U: show synchronous_standby_names;
+2U: show synchronous_standby_names;
 synchronous_standby_names
 -------------------------
 *                        

src/test/isolation2/sql/segwalrep/fts_unblock_primary.sql

@@ -44,8 +44,7 @@ select content, role, preferred_role, mode, status from gp_segment_configuration
 
 -- synchronous_standby_names should be set to '*' by default on primary 2, since
 -- we have a working/sync'd mirror
--- XXX We assume primary 2 is DBID 4, which may not be the case.
-4U: show synchronous_standby_names;
+2U: show synchronous_standby_names;
 
 -- create table and show commits are not blocked
 create table fts_unblock_primary (a int) distributed by (a);
@@ -78,7 +77,7 @@ select content, role, preferred_role, mode, status from gp_segment_configuration
 2<:
 
 -- synchronous_standby_names should now be empty on the primary
-4U: show synchronous_standby_names;
+2U: show synchronous_standby_names;
 
 -- bring the mirror back up and see primary s/u and mirror s/u
 -1U: select pg_ctl((select datadir from gp_segment_configuration c where c.role='m' and c.content=2), 'start', (select port from gp_segment_configuration where content = 2 and preferred_role = 'm'), 2);
@@ -89,4 +88,4 @@ select content, role, preferred_role, mode, status from gp_segment_configuration
 insert into fts_unblock_primary select i from generate_series(1,10)i;
 
 -- synchronous_standby_names should be back to its original value on the primary
-4U: show synchronous_standby_names;
+2U: show synchronous_standby_names;