Once again allow LWLocks to be used within DSM segments.

Prior to commit 7882c3b0b9, it was
possible to use LWLocks within DSM segments, but that commit broke
this use case by switching from a doubly linked list to a circular
linked list.  Switch back, using a new bit of general infrastructure
for maintaining lists of PGPROCs.

Thomas Munro, reviewed by me.

src/include/storage/lwlock.h

@@ -18,7 +18,7 @@
 #error "lwlock.h may not be included from frontend code"
 #endif
 
-#include "lib/ilist.h"
+#include "storage/proclist_types.h"
 #include "storage/s_lock.h"
 #include "port/atomics.h"
 
@@ -59,7 +59,7 @@ typedef struct LWLock
 {
 	uint16		tranche;		/* tranche ID */
 	pg_atomic_uint32 state;		/* state of exclusive/nonexclusive lockers */
-	dlist_head	waiters;		/* list of waiting PGPROCs */
+	proclist_head waiters;		/* list of waiting PGPROCs */
 #ifdef LOCK_DEBUG
 	pg_atomic_uint32 nwaiters;	/* number of waiters */
 	struct PGPROC *owner;		/* last exclusive owner of the lock */