Add PQresultMemorySize function to report allocated size of a PGresult.

This number can be useful for application memory management, and the
overhead to track it seems pretty trivial.

Lars Kanis, reviewed by Pavel Stehule, some mods by me

Discussion: https://postgr.es/m/fa16a288-9685-14f2-97c8-b8ac84365a4f@greiz-reinsdorf.de

src/interfaces/libpq/libpq-int.h

@@ -208,6 +208,8 @@ struct pg_result
 	PGresult_data *curBlock;	/* most recently allocated block */
 	int			curOffset;		/* start offset of free space in block */
 	int			spaceLeft;		/* number of free bytes remaining in block */
+
+	size_t		memorySize;		/* total space allocated for this PGresult */
 };
 
 /* PGAsyncStatusType defines the state of the query-execution state machine */