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/utils/memutils.h

@@ -155,6 +155,11 @@ extern MemoryContext SlabContextCreate(MemoryContext parent,
 				  Size blockSize,
 				  Size chunkSize);
 
+/* generation.c */
+extern MemoryContext GenerationContextCreate(MemoryContext parent,
+				 const char *name,
+				 Size blockSize);
+
 /*
  * Recommended default alloc parameters, suitable for "ordinary" contexts
  * that might hold quite a lot of data.