Add support for coordinating record typmods among parallel workers.

Tuples can have type RECORDOID and a typmod number that identifies a blessed
TupleDesc in a backend-private cache.  To support the sharing of such tuples
through shared memory and temporary files, provide a typmod registry in
shared memory.

To achieve that, introduce per-session DSM segments, created on demand when a
backend first runs a parallel query.  The per-session DSM segment has a
table-of-contents just like the per-query DSM segment, and initially the
contents are a shared record typmod registry and a DSA area to provide the
space it needs to grow.

State relating to the current session is accessed via a Session object
reached through global variable CurrentSession that may require significant
redesign further down the road as we figure out what else needs to be shared
or remodelled.

Author: Thomas Munro
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAEepm=0ZtQ-SpsgCyzzYpsXS6e=kZWqk3g5Ygn3MDV7A8dabUA@mail.gmail.com

src/backend/utils/init/postinit.c

@@ -21,6 +21,7 @@
 
 #include "access/heapam.h"
 #include "access/htup_details.h"
+#include "access/session.h"
 #include "access/sysattr.h"
 #include "access/xact.h"
 #include "access/xlog.h"
@@ -1027,6 +1028,9 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
 	/* initialize client encoding */
 	InitializeClientEncoding();
 
+	/* Initialize this backend's session state. */
+	InitializeSession();
+
 	/* report this backend in the PgBackendStatus array */
 	if (!bootstrap)
 		pgstat_bestart();