MDEV-13059 XtraDB hangs on Windows due to failing to release block->lock X-latch

in innodb_read_only mode.

The reason for the hang is that there was no notification received about
completed read io. File handles are bound to completion_port, and there
were no background "write" threads that would be waiting on completion_port,
only 2 "read" threads waiting on read_completion_port were active.

The fix is to use a single IO completion port for all IOs, if
innodb_read_only is set.

mysql-test/suite/innodb/t/row_format_redundant.test

@@ -2,16 +2,6 @@
 # Embedded mode doesn't allow restarting
 --source include/not_embedded.inc
 
-# MDEV-13059 XtraDB hangs on Windows due to failing to release
-# block->lock X-latch in innodb_read_only mode
-if (`SELECT count(*) FROM information_schema.plugins WHERE
-      plugin_name = 'innodb' AND plugin_status = 'active' AND
-      plugin_description LIKE '%xtradb%'`){
-    if (`SELECT @@version_compile_os IN ('Win32','Win64','Windows')`) {
-       skip MDEV-13059 XtraDB hangs on Windows in innodb_read_only mode;
-    }
-}
-
 create table t1 (a int not null, d varchar(15) not null, b
 varchar(198) not null, c char(156),
 fulltext ftsic(c)) engine=InnoDB

storage/xtradb/os/os0file.cc

@@ -4163,7 +4163,7 @@ os_aio_init(
 #ifdef _WIN32
 	ut_a(completion_port == 0 && read_completion_port == 0);
 	completion_port = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);
-	read_completion_port = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);
+	read_completion_port = srv_read_only_mode? completion_port : CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);
 	ut_a(completion_port && read_completion_port);
 #endif