Fix inadequacies in recently added wait events

In commit 9915de6c1c, we introduced a new wait point for replication
slots and incorrectly labelled it as wait event PG_WAIT_LOCK.  That's
wrong, so invent an appropriate new wait event instead, and document it
properly.

While at it, fix numerous other problems in the vicinity:
- two different walreceiver wait events were being mixed up in a single
  wait event (which wasn't documented either); split it out so that they
  can be distinguished, and document the new events properly.

- ParallelBitmapPopulate was documented but didn't exist.

- ParallelBitmapScan was not documented (I think this should be called
  "ParallelBitmapScanInit" instead.)

- Logical replication wait events weren't documented

- various symbols had been added in dartboard order in various places.
  Put them in alphabetical order instead, as was originally intended.

Discussion: https://postgr.es/m/20170808181131.mu4fjepuh5m75cyq@alvherre.pgsql

src/include/pgstat.h

@@ -759,15 +759,15 @@ typedef enum
 	WAIT_EVENT_BGWRITER_HIBERNATE,
 	WAIT_EVENT_BGWRITER_MAIN,
 	WAIT_EVENT_CHECKPOINTER_MAIN,
+	WAIT_EVENT_LOGICAL_LAUNCHER_MAIN,
+	WAIT_EVENT_LOGICAL_APPLY_MAIN,
 	WAIT_EVENT_PGSTAT_MAIN,
 	WAIT_EVENT_RECOVERY_WAL_ALL,
 	WAIT_EVENT_RECOVERY_WAL_STREAM,
 	WAIT_EVENT_SYSLOGGER_MAIN,
 	WAIT_EVENT_WAL_RECEIVER_MAIN,
 	WAIT_EVENT_WAL_SENDER_MAIN,
-	WAIT_EVENT_WAL_WRITER_MAIN,
-	WAIT_EVENT_LOGICAL_LAUNCHER_MAIN,
-	WAIT_EVENT_LOGICAL_APPLY_MAIN
+	WAIT_EVENT_WAL_WRITER_MAIN
 } WaitEventActivity;
 
 /* ----------
@@ -782,9 +782,10 @@ typedef enum
 {
 	WAIT_EVENT_CLIENT_READ = PG_WAIT_CLIENT,
 	WAIT_EVENT_CLIENT_WRITE,
+	WAIT_EVENT_LIBPQWALRECEIVER_CONNECT,
+	WAIT_EVENT_LIBPQWALRECEIVER_RECEIVE,
 	WAIT_EVENT_SSL_OPEN_SERVER,
 	WAIT_EVENT_WAL_RECEIVER_WAIT_START,
-	WAIT_EVENT_LIBPQWALRECEIVER,
 	WAIT_EVENT_WAL_SENDER_WAIT_WAL,
 	WAIT_EVENT_WAL_SENDER_WRITE_DATA
 } WaitEventClient;
@@ -802,6 +803,8 @@ typedef enum
 	WAIT_EVENT_BGWORKER_STARTUP,
 	WAIT_EVENT_BTREE_PAGE,
 	WAIT_EVENT_EXECUTE_GATHER,
+	WAIT_EVENT_LOGICAL_SYNC_DATA,
+	WAIT_EVENT_LOGICAL_SYNC_STATE_CHANGE,
 	WAIT_EVENT_MQ_INTERNAL,
 	WAIT_EVENT_MQ_PUT_MESSAGE,
 	WAIT_EVENT_MQ_RECEIVE,
@@ -809,10 +812,9 @@ typedef enum
 	WAIT_EVENT_PARALLEL_FINISH,
 	WAIT_EVENT_PARALLEL_BITMAP_SCAN,
 	WAIT_EVENT_PROCARRAY_GROUP_UPDATE,
+	WAIT_EVENT_REPLICATION_SLOT_DROP,
 	WAIT_EVENT_SAFE_SNAPSHOT,
-	WAIT_EVENT_SYNC_REP,
-	WAIT_EVENT_LOGICAL_SYNC_DATA,
-	WAIT_EVENT_LOGICAL_SYNC_STATE_CHANGE
+	WAIT_EVENT_SYNC_REP
 } WaitEventIPC;
 
 /* ----------