Move Timestamp/Interval typedefs and basic macros into datatype/timestamp.h.

As per my recent proposal, this refactors things so that these typedefs and
macros are available in a header that can be included in frontend-ish code.
I also changed various headers that were undesirably including
utils/timestamp.h to include datatype/timestamp.h instead.  Unsurprisingly,
this showed that half the system was getting utils/timestamp.h by way of
xlog.h.

No actual code changes here, just header refactoring.

src/backend/commands/analyze.c

@@ -48,6 +48,7 @@
 #include "utils/pg_rusage.h"
 #include "utils/syscache.h"
 #include "utils/tuplesort.h"
+#include "utils/timestamp.h"
 #include "utils/tqual.h"
 
 

src/backend/commands/async.c

@@ -132,6 +132,7 @@
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/ps_status.h"
+#include "utils/timestamp.h"
 
 
 /*

src/backend/commands/prepare.c

@@ -31,6 +31,7 @@
 #include "tcop/utility.h"
 #include "utils/builtins.h"
 #include "utils/snapmgr.h"
+#include "utils/timestamp.h"
 
 
 /*

src/backend/commands/user.c

@@ -33,6 +33,7 @@
 #include "utils/builtins.h"
 #include "utils/fmgroids.h"
 #include "utils/syscache.h"
+#include "utils/timestamp.h"
 #include "utils/tqual.h"
 
 /* Potentially set by contrib/pg_upgrade_support functions */

src/backend/commands/vacuumlazy.c

@@ -53,6 +53,7 @@
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"
 #include "utils/pg_rusage.h"
+#include "utils/timestamp.h"
 #include "utils/tqual.h"
 
 

src/backend/commands/variable.c

@@ -26,6 +26,7 @@
 #include "utils/builtins.h"
 #include "utils/syscache.h"
 #include "utils/snapmgr.h"
+#include "utils/timestamp.h"
 #include "mb/pg_wchar.h"
 
 /*