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.