core: Add internal transfer state management

core: Add internal transfer state management

This patch adds some new flags to keep track of transfer state.
These flags are used to properly handle transfers that are on
the flying_transfers list for devices that are disconnected.

The motivation for this patch is to release the requirement of
holding the flying_transfers_lock for the duration of a call to
libusb_submit_transfer(). Holding this lock is the simplest and
safest way to submit a transfer, but it has performance impacts
as it serializes transfer submission for a given context.

With proper transfer state management, the library can handle a
device disconnect without needing to prevent multiple transfers
from being concurrently submitted.

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