Refactor threadpool (move common logic out of platform specific files into shared code)

In light of adding priorities to threadpool (MDEV-10297), it would be good to have common code/logic shared by windows and unix implementations.

Specifically, it would be very helpful to have a single callback function, that is called when new connection is queued or when client IO arrives. That callback would also do a proper cleanup in case of error/quit, for both THD and scheduler specific structures.

The code would be similar to handle_event()
https://github.com/MariaDB/server/blob/10.1/sql/threadpool_unix.cc#L1437

But, for this code to be in threadpool_common.cc, we need to expose some platform/imple functionality (start_io / set_wait_timeout, connection_t struct) somehow.