Fix various common mispellings.

Mostly these are just comments but there are a few in documentation
and a handful in code and tests. Hopefully this doesn't cause too much
unnecessary pain for backpatching. I relented from some of the most
common like "thru" for that reason. The rest don't seem numerous
enough to cause problems.

Thanks to Kevin Lyda's tool https://pypi.python.org/pypi/misspellings

src/backend/replication/logical/reorderbuffer.c

@@ -563,7 +563,7 @@ ReorderBufferTXNByXid(ReorderBuffer *rb, TransactionId xid, bool create,
 		}
 
 		/*
-		 * cached as non-existant, and asked not to create? Then nothing else
+		 * cached as non-existent, and asked not to create? Then nothing else
 		 * to do.
 		 */
 		if (!create)

src/backend/replication/walsender.c

@@ -747,7 +747,7 @@ StartReplication(StartReplicationCmd *cmd)
  *
  * Inside the walsender we can do better than logical_read_local_xlog_page,
  * which has to do a plain sleep/busy loop, because the walsender's latch gets
- * set everytime WAL is flushed.
+ * set every time WAL is flushed.
  */
 static int
 logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int reqLen,