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.