Move each SLRU's lwlocks to a separate tranche.

This makes it significantly easier to identify these lwlocks in
LWLOCK_STATS or Trace_lwlocks output.  It's also arguably better
from a modularity standpoint, since lwlock.c no longer needs to
know anything about the LWLock needs of the higher-level SLRU
facility.

Ildus Kurbangaliev, reviewd by Álvaro Herrera and by me.

src/backend/access/transam/clog.c

@@ -456,7 +456,7 @@ void
 CLOGShmemInit(void)
 {
 	ClogCtl->PagePrecedes = CLOGPagePrecedes;
-	SimpleLruInit(ClogCtl, "CLOG Ctl", CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE,
+	SimpleLruInit(ClogCtl, "clog", CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE,
 				  CLogControlLock, "pg_clog");
 }