Generational memory allocator

Add new style of memory allocator, known as Generational
appropriate for use in cases where memory is allocated
and then freed in roughly oldest first order (FIFO).

Use new allocator for logical decoding’s reorderbuffer
to significantly reduce memory usage and improve performance.

Author: Tomas Vondra
Reviewed-by: Simon Riggs

src/include/nodes/nodes.h

@@ -274,6 +274,7 @@ typedef enum NodeTag
 	T_MemoryContext,
 	T_AllocSetContext,
 	T_SlabContext,
+	T_GenerationContext,
 
 	/*
 	 * TAGS FOR VALUE NODES (value.h)