Log when a BRIN autosummarization request fails

Autovacuum's 'workitem' request queue is of limited size, so requests
can fail if they arrive more quickly than autovacuum can process them.
Emit a log message when this happens, to provide better visibility of
this.

Backpatch to 10.  While this represents an API change for
AutoVacuumRequestWork, that function is not yet prepared to deal with
external modules calling it, so there doesn't seem to be any risk (other
than log spam, that is.)

Author: Masahiko Sawada
Reviewed-by: Fabrízio Mello, Ildar Musin, Álvaro Herrera
Discussion: https://postgr.es/m/CAD21AoB1HrQhp6_4rTyHN5kWEJCEsG8YzsjZNt-ctoXSn5Uisw@mail.gmail.com

src/backend/postmaster/autovacuum.c

@@ -3227,12 +3227,14 @@ AutoVacuumingActive(void)
 
 /*
  * Request one work item to the next autovacuum run processing our database.
+ * Return false if the request can't be recorded.
  */
-void
+bool
 AutoVacuumRequestWork(AutoVacuumWorkItemType type, Oid relationId,
 					  BlockNumber blkno)
 {
 	int			i;
+	bool		result = false;
 
 	LWLockAcquire(AutovacuumLock, LW_EXCLUSIVE);
 
@@ -3252,12 +3254,15 @@ AutoVacuumRequestWork(AutoVacuumWorkItemType type, Oid relationId,
 		workitem->avw_database = MyDatabaseId;
 		workitem->avw_relation = relationId;
 		workitem->avw_blockNumber = blkno;
+		result = true;
 
 		/* done */
 		break;
 	}
 
 	LWLockRelease(AutovacuumLock);
+
+	return result;
 }
 
 /*