MDEV-77 - possible deadlock in XtraDB async io subsystem on Windows.
MDEV-77 - possible deadlock in XtraDB async io subsystem on Windows.

Split IO threads into ones that handle only read completion and ones that handle only write completion, as it was originally done, but got lost with "completion port" patch. The reason we need to have dedicated read and dedicated write threads is that read completion routine can block waiting for write io to complete, and in rare cases where all io threads are handling async reads, it can deadlock.

davidl-zend/zenddbi,ollie314/server,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,ollie314/server,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,flynn1973/mariadb-aix,ollie314/server,ollie314/server,natsys/mariadb_10.2,davidl-zend/zenddbi,natsys/mariadb_10.2,davidl-zend/zenddbi,ollie314/server,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,davidl-zend/zenddbi,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,flynn1973/mariadb-aix,natsys/mariadb_10.2,flynn1973/mariadb-aix,slanterns/server,davidl-zend/zenddbi