Rethink node-level representation of partial-aggregation modes.

The original coding had three separate booleans representing partial
aggregation behavior, which was confusing, unreadable, and error-prone,
not least because the booleans weren't always listed in the same order.
It was also inadequate for the allegedly-desirable future extension to
support intermediate partial aggregation, because we'd need separate
markers for serialization and deserialization in such a case.

Merge these bools into an enum "AggSplit" to provide symbolic names for
the supported operating modes (and document what those are).  By assigning
the values of the enum constants carefully, we can treat AggSplit values
as options bitmasks so that tests of what to do aren't noticeably more
expensive than before.

While at it, get rid of Aggref.aggoutputtype.  That's not needed since
commit 59a3795c2 got rid of setrefs.c's special-purpose Aggref comparison
code, and it likewise seemed more confusing than helpful.

Assorted comment cleanup as well (there's still more that I want to do
in that line).

catversion bump for change in Aggref node contents.  Should be the last
one for partial-aggregation changes.

Discussion: <29309.1466699160@sss.pgh.pa.us>

src/include/nodes/nodes.h

@@ -694,6 +694,36 @@ typedef enum AggStrategy
 	AGG_HASHED					/* grouped agg, use internal hashtable */
 } AggStrategy;
 
+/*
+ * AggSplit -
+ *	  splitting (partial aggregation) modes for Agg plan nodes
+ *
+ * This is needed in both plannodes.h and relation.h, so put it here...
+ */
+
+/* Primitive options supported by nodeAgg.c: */
+#define AGGSPLITOP_COMBINE		0x01	/* substitute combinefn for transfn */
+#define AGGSPLITOP_SKIPFINAL	0x02	/* skip finalfn, return state as-is */
+#define AGGSPLITOP_SERIALIZE	0x04	/* apply serializefn to output */
+#define AGGSPLITOP_DESERIALIZE	0x08	/* apply deserializefn to input */
+
+/* Supported operating modes (i.e., useful combinations of these options): */
+typedef enum AggSplit
+{
+	/* Basic, non-split aggregation: */
+	AGGSPLIT_SIMPLE = 0,
+	/* Initial phase of partial aggregation, with serialization: */
+	AGGSPLIT_INITIAL_SERIAL = AGGSPLITOP_SKIPFINAL | AGGSPLITOP_SERIALIZE,
+	/* Final phase of partial aggregation, with deserialization: */
+	AGGSPLIT_FINAL_DESERIAL = AGGSPLITOP_COMBINE | AGGSPLITOP_DESERIALIZE
+} AggSplit;
+
+/* Test whether an AggSplit value selects each primitive option: */
+#define DO_AGGSPLIT_COMBINE(as)		(((as) & AGGSPLITOP_COMBINE) != 0)
+#define DO_AGGSPLIT_SKIPFINAL(as)	(((as) & AGGSPLITOP_SKIPFINAL) != 0)
+#define DO_AGGSPLIT_SERIALIZE(as)	(((as) & AGGSPLITOP_SERIALIZE) != 0)
+#define DO_AGGSPLIT_DESERIALIZE(as) (((as) & AGGSPLITOP_DESERIALIZE) != 0)
+
 /*
  * SetOpCmd and SetOpStrategy -
  *	  overall semantics and execution strategies for SetOp plan nodes