After the merge of VIO stuff from MySQL 5.6, there were some bugs left
in the non-blocking client library:

 - vio_io_wait() was introduced without any support for non-blocking operation,
   so async queries could turn into sync.

 - Timeouts were changed to milliseconds, but this was not reflected in the
   non-blocking API, also semantics was changed so signed -1 was used for
   "no timeout" rather than unsigned 0.

Fix by implementing and using my_io_wait_async() in the non-blocking case. And
by introducing a new mysql_get_timeout_value_ms() API function that provides
the timeout with millisecond granularity. The old mysql_get_timeout_value()
is kept and fixed to work correctly, converting the timeout to whole seconds.