Increase the number of buffer mapping partitions to 128.

Testing by Amit Kapila, Andres Freund, and myself, with and without
other patches that also aim to improve scalability, seems to indicate
that this change is a significant win over the current value and over
smaller values such as 64.  It's not clear how high we can push this
value before it starts to have negative side-effects elsewhere, but
going this far looks OK.

src/backend/storage/lmgr/lwlock.c

@@ -73,11 +73,11 @@ static LWLockTranche MainLWLockTranche;
 
 /*
  * We use this structure to keep track of locked LWLocks for release
- * during error recovery.  The maximum size could be determined at runtime
- * if necessary, but it seems unlikely that more than a few locks could
- * ever be held simultaneously.
+ * during error recovery.  Normally, only a few will be held at once, but
+ * occasionally the number can be much higher; for example, the pg_buffercache
+ * extension locks all buffer partitions simultaneously.
  */
-#define MAX_SIMUL_LWLOCKS	100
+#define MAX_SIMUL_LWLOCKS	200
 
 static int	num_held_lwlocks = 0;
 static LWLock *held_lwlocks[MAX_SIMUL_LWLOCKS];

src/include/storage/lwlock.h

@@ -136,7 +136,7 @@ extern PGDLLIMPORT LWLockPadded *MainLWLockArray;
  */
 
 /* Number of partitions of the shared buffer mapping hashtable */
-#define NUM_BUFFER_PARTITIONS  16
+#define NUM_BUFFER_PARTITIONS  128
 
 /* Number of partitions the shared lock tables are divided into */
 #define LOG2_NUM_LOCK_PARTITIONS  4