Provide interim statistics while in mid-checkpoint.

Re-implements similar functionality in 9.1 and previously which
was removed during split of checkpointer and bgwriter.

Requested/spotted by Magnus Hagander

Author: simonat2ndQuadrant

src/backend/postmaster/checkpointer.c

@@ -708,6 +708,11 @@ CheckpointWriteDelay(int flags, double progress)
 
 		CheckArchiveTimeout();
 
+		/*
+		 * Report interim activity statistics to the stats collector.
+		 */
+		pgstat_send_bgwriter();
+
 		/*
 		 * This sleep used to be connected to bgwriter_delay, typically 200ms.
 		 * That resulted in more frequent wakeups if not much work to do.