Make xact.h usable in frontend.

xact.h included utils/datetime.h, which cannot be used in the frontend
(it includes fmgr.h, which needs Datum). But xact.h only needs the
definition of TimestampTz from it, which is available directly in
datatypes/timestamp.h. Change xact.h to include that instead of
utils/datetime.h, so that it can be used in client programs.

src/backend/nodes/params.c

@@ -16,6 +16,7 @@
 #include "postgres.h"
 
 #include "access/xact.h"
+#include "fmgr.h"
 #include "mb/stringinfo_mb.h"
 #include "nodes/params.h"
 #include "parser/parse_node.h"

src/backend/utils/time/snapmgr.c

@@ -53,6 +53,7 @@
 #include "access/xact.h"
 #include "access/xlog.h"
 #include "catalog/catalog.h"
+#include "datatype/timestamp.h"
 #include "lib/pairingheap.h"
 #include "miscadmin.h"
 #include "storage/predicate.h"
@@ -67,6 +68,7 @@
 #include "utils/resowner_private.h"
 #include "utils/snapmgr.h"
 #include "utils/syscache.h"
+#include "utils/timestamp.h"
 
 
 /*

src/include/access/xact.h

@@ -16,11 +16,11 @@
 
 #include "access/transam.h"
 #include "access/xlogreader.h"
+#include "datatype/timestamp.h"
 #include "lib/stringinfo.h"
 #include "nodes/pg_list.h"
 #include "storage/relfilenode.h"
 #include "storage/sinval.h"
-#include "utils/datetime.h"
 
 /*
  * Maximum size of Global Transaction ID (including '\0').