Change MAX_IO_CONCURRENCY to be power of 2

Author: knizhnik

src/include/storage/bufmgr.h

@@ -86,8 +86,8 @@ extern PGDLLIMPORT int NLocBuffer;
 extern PGDLLIMPORT Block *LocalBufferBlockPointers;
 extern PGDLLIMPORT int32 *LocalRefCount;
 
-/* upper limit for effective_io_concurrency */
-#define MAX_IO_CONCURRENCY 1000
+/* upper limit for effective_io_concurrency (better to he power of 2) */
+#define MAX_IO_CONCURRENCY 1024
 
 /* special block number for ReadBuffer() */
 #define P_NEW	InvalidBlockNumber	/* grow the file to get a new page */