Clean up the rather historically encumbered interface to now() and

current time: provide a GetCurrentTimestamp() function that returns
current time in the form of a TimestampTz, instead of separate time_t
and microseconds fields.  This is what all the callers really want
anyway, and it eliminates low-level dependencies on AbsoluteTime,
which is a deprecated datatype that will have to disappear eventually.

contrib/btree_gist/btree_ts.c

@@ -1,6 +1,9 @@
 #include "btree_gist.h"
 #include "btree_utils_num.h"
 
+#include "utils/datetime.h"
+
+
 typedef struct
 {
 	Timestamp	lower;