Back-patch addition of the ALLOCSET_FOO_SIZES macros.

These macros were originally added in commit ea268cdc, and back-patched
into 9.6 before 9.6.0.  However, some extensions would like to use them
in older branches, and there seems no harm in providing them.  So add
them to all supported branches.  Per suggestions from Christoph Berg and
Andres Freund.

Discussion: https://postgr.es/m/20181012170355.bhxi273skjt6sag4@alap3.anarazel.de

src/include/utils/memutils.h

@@ -132,14 +132,26 @@ extern MemoryContext AllocSetContextCreate(MemoryContext parent,
 #define ALLOCSET_DEFAULT_MINSIZE   0
 #define ALLOCSET_DEFAULT_INITSIZE  (8 * 1024)
 #define ALLOCSET_DEFAULT_MAXSIZE   (8 * 1024 * 1024)
+#define ALLOCSET_DEFAULT_SIZES \
+	ALLOCSET_DEFAULT_MINSIZE, ALLOCSET_DEFAULT_INITSIZE, ALLOCSET_DEFAULT_MAXSIZE
 
 /*
- * Recommended alloc parameters for "small" contexts that are not expected
+ * Recommended alloc parameters for "small" contexts that are never expected
  * to contain much data (for example, a context to contain a query plan).
  */
 #define ALLOCSET_SMALL_MINSIZE	 0
 #define ALLOCSET_SMALL_INITSIZE  (1 * 1024)
 #define ALLOCSET_SMALL_MAXSIZE	 (8 * 1024)
+#define ALLOCSET_SMALL_SIZES \
+	ALLOCSET_SMALL_MINSIZE, ALLOCSET_SMALL_INITSIZE, ALLOCSET_SMALL_MAXSIZE
+
+/*
+ * Recommended alloc parameters for contexts that should start out small,
+ * but might sometimes grow big.
+ */
+#define ALLOCSET_START_SMALL_SIZES \
+	ALLOCSET_SMALL_MINSIZE, ALLOCSET_SMALL_INITSIZE, ALLOCSET_DEFAULT_MAXSIZE
+
 
 /*
  * Threshold above which a request in an AllocSet context is certain to be