needed to move to a new structure MYSQLND_STMT. Makes
the code cleaner and less error-prone.
Also fix PDO/MySQL which directly touch mysqlnd internals
instead of using API calls.
Clearly separated fetching (physical reading) from decoding phases (data
interpretation). Threaded fetching added but disabled as needs more work for
Windows. For Linux needs some touches to add pthreads if this is enabled,
probably with a compile-time switch.
The code reorganisation makes it easy to add also async API, similar to
cURL's one.
