add lock to fix.py when processing incoming order messages

[MikeHathaway]

Attempting to resolve an issue wherein an order is placed with a successful response, immediately followed by a second response message that the successfully placed order would violate post only requirements. This edge case should be currently handled by checking messge.get(b'5001') is not None, since all such INVALID ALO or cancellation rejection messages contain this field.

Since the shutdowns are still occuring, even with this check on both sync_orders and wait_for_order_processing_response, this is making me think there is an asynchronicity issue from these reject messages not being queued prior to the unnecessary attempted cancellation.

[codecov-io]

Codecov Report

Merging #205 (a6665f4) into master (79c1f72) will not change coverage. The diff coverage is n/a.

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