Windows: Use I/O completion ports for transfers

Windows: Use I/O completion ports for transfers

As a first step in removing the Windows poll() emulation, switch the
transfers to use an I/O completion port. A dedicated per-context thread
will wait on the I/O completion port and report transfer completions
using usbi_signal_transfer_completion(). This enables the complete
removal of the handle_events() function for the Windows backend and
removes the notion of one "file descriptor" per transfer.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>