Rename assorted LWLock tranches.

Choose names that fit into the conventions for wait event names
(particularly, that multi-word names are in the style MultiWordName)
and hopefully convey more information to non-hacker users than the
previous names did.

Also rename SerializablePredicateLockListLock to
SerializablePredicateListLock; the old name was long enough to cause
table formatting problems, plus the double occurrence of "Lock" seems
confusing/error-prone.

Also change a couple of particularly opaque LWLock field names.

Discussion: https://postgr.es/m/28683.1589405363@sss.pgh.pa.us

src/backend/storage/buffer/buf_init.c

@@ -132,7 +132,7 @@ InitBufferPool(void)
 							 LWTRANCHE_BUFFER_CONTENT);
 
 			LWLockInitialize(BufferDescriptorGetIOLock(buf),
-							 LWTRANCHE_BUFFER_IO_IN_PROGRESS);
+							 LWTRANCHE_BUFFER_IO);
 		}
 
 		/* Correct last entry of linked list */