Use one transaction while reading postgres.bki, not one per line.

AFAICT, the only actual benefit of closing a bootstrap transaction
is to reclaim transient memory.  We can do that a lot more cheaply
by just doing a MemoryContextReset on a suitable context.  This
gets the runtime of the "bootstrap" phase of initdb down to the
point where, at least by eyeball, it's quite negligible compared
to the rest of the phases.  Per discussion with Andres Freund.

Discussion: https://postgr.es/m/9244.1492106743@sss.pgh.pa.us

src/backend/bootstrap/bootparse.y

@@ -22,7 +22,6 @@
 #include "access/htup.h"
 #include "access/itup.h"
 #include "access/tupdesc.h"
-#include "access/xact.h"
 #include "bootstrap/bootstrap.h"
 #include "catalog/catalog.h"
 #include "catalog/heap.h"
@@ -49,6 +48,7 @@
 #include "storage/off.h"
 #include "storage/smgr.h"
 #include "tcop/dest.h"
+#include "utils/memutils.h"
 #include "utils/rel.h"
 
 
@@ -63,19 +63,27 @@
 #define YYMALLOC palloc
 #define YYFREE   pfree
 
+static MemoryContext per_line_ctx = NULL;
+
 static void
 do_start(void)
 {
-	StartTransactionCommand();
-	elog(DEBUG4, "start transaction");
+	Assert(CurrentMemoryContext == CurTransactionContext);
+	/* First time through, create the per-line working context */
+	if (per_line_ctx == NULL)
+		per_line_ctx = AllocSetContextCreate(CurTransactionContext,
+											 "bootstrap per-line processing",
+											 ALLOCSET_DEFAULT_SIZES);
+	MemoryContextSwitchTo(per_line_ctx);
 }
 
 
 static void
 do_end(void)
 {
-	CommitTransactionCommand();
-	elog(DEBUG4, "commit transaction");
+	/* Reclaim memory allocated while processing this line */
+	MemoryContextSwitchTo(CurTransactionContext);
+	MemoryContextReset(per_line_ctx);
 	CHECK_FOR_INTERRUPTS();		/* allow SIGINT to kill bootstrap run */
 	if (isatty(0))
 	{

src/backend/bootstrap/bootstrap.c

@@ -18,6 +18,7 @@
 #include <signal.h>
 
 #include "access/htup_details.h"
+#include "access/xact.h"
 #include "bootstrap/bootstrap.h"
 #include "catalog/index.h"
 #include "catalog/pg_collation.h"
@@ -496,7 +497,9 @@ BootstrapModeMain(void)
 	/*
 	 * Process bootstrap input.
 	 */
+	StartTransactionCommand();
 	boot_yyparse();
+	CommitTransactionCommand();
 
 	/*
 	 * We should now know about all mapped relations, so it's okay to write