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/include/access/gist_private.h

@@ -16,6 +16,7 @@
 
 #include "access/gist.h"
 #include "access/itup.h"
+#include "fmgr.h"
 #include "storage/bufmgr.h"
 #include "storage/buffile.h"
 #include "utils/rbtree.h"

src/include/access/xlog.h

@@ -13,10 +13,10 @@
 
 #include "access/rmgr.h"
 #include "access/xlogdefs.h"
+#include "datatype/timestamp.h"
 #include "lib/stringinfo.h"
 #include "storage/buf.h"
 #include "utils/pg_crc.h"
-#include "utils/timestamp.h"
 
 /*
  * The overall layout of an XLOG record is:

src/include/access/xlog_internal.h

@@ -17,6 +17,8 @@
 #define XLOG_INTERNAL_H
 
 #include "access/xlog.h"
+#include "fmgr.h"
+#include "pgtime.h"
 #include "storage/block.h"
 #include "storage/relfilenode.h"