Fix memory leaks in PL/Python.

Previously, plpython was in the habit of allocating a lot of stuff in
TopMemoryContext, and it was very slipshod about making sure that stuff
got cleaned up; in particular, use of TopMemoryContext as fn_mcxt for
function calls represents an unfixable leak, since we generally don't
know what the called function might have allocated in fn_mcxt.  This
results in session-lifespan leakage in certain usage scenarios, as for
example in a case reported by Ed Behn back in July.

To fix, get rid of all the retail allocations in TopMemoryContext.
All long-lived allocations are now made in sub-contexts that are
associated with specific objects (either pl/python procedures, or
Python-visible objects such as cursors and plans).  We can clean these
up when the associated object is deleted.

I went so far as to get rid of PLy_malloc completely.  There were a
couple of places where it could still have been used safely, but on
the whole it was just an invitation to bad coding.

Haribabu Kommi, based on a draft patch by Heikki Linnakangas;
some further work by me

src/pl/plpython/plpy_util.c

@@ -17,42 +17,6 @@
 #include "plpy_elog.h"
 
 
-void *
-PLy_malloc(size_t bytes)
-{
-	/* We need our allocations to be long-lived, so use TopMemoryContext */
-	return MemoryContextAlloc(TopMemoryContext, bytes);
-}
-
-void *
-PLy_malloc0(size_t bytes)
-{
-	void	   *ptr = PLy_malloc(bytes);
-
-	MemSet(ptr, 0, bytes);
-	return ptr;
-}
-
-char *
-PLy_strdup(const char *str)
-{
-	char	   *result;
-	size_t		len;
-
-	len = strlen(str) + 1;
-	result = PLy_malloc(len);
-	memcpy(result, str, len);
-
-	return result;
-}
-
-/* define this away */
-void
-PLy_free(void *ptr)
-{
-	pfree(ptr);
-}
-
 /*
  * Convert a Python unicode object to a Python string/bytes object in
  * PostgreSQL server encoding.  Reference ownership is passed to the