Consistently declare timestamp variables as TimestampTz.

Twiddle the replication-related code so that its timestamp variables
are declared TimestampTz, rather than the uninformative "int64" that
was previously used for meant-to-be-always-integer timestamps.
This resolves the int64-vs-TimestampTz declaration inconsistencies
introduced by commit 7c030783a, though in the opposite direction to
what was originally suggested.

This required including datatype/timestamp.h in a couple more places
than before.  I decided it would be a good idea to slim down that
header by not having it pull in <float.h> etc, as those headers are
no longer at all relevant to its purpose.  Unsurprisingly, a small number
of .c files turn out to have been depending on those inclusions, so add
them back in the .c files as needed.

Discussion: https://postgr.es/m/26788.1487455319@sss.pgh.pa.us
Discussion: https://postgr.es/m/27694.1487456324@sss.pgh.pa.us

src/include/utils/snapshot.h

@@ -15,6 +15,7 @@
 
 #include "access/htup.h"
 #include "access/xlogdefs.h"
+#include "datatype/timestamp.h"
 #include "lib/pairingheap.h"
 #include "storage/buf.h"
 
@@ -107,7 +108,7 @@ typedef struct SnapshotData
 	uint32		regd_count;		/* refcount on RegisteredSnapshots */
 	pairingheap_node ph_node;	/* link in the RegisteredSnapshots heap */
 
-	int64		whenTaken;		/* timestamp when snapshot was taken */
+	TimestampTz whenTaken;		/* timestamp when snapshot was taken */
 	XLogRecPtr	lsn;			/* position in the WAL stream when taken */
 } SnapshotData;