Fix #481, simplify and consolidate state machines

[jphickey]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to GSFC-SoftwareRelease@mail.nasa.gov and copied cfs-program@lists.nasa.gov.

Describe the contribution
Rework state machines such that all state changes are performed in the tick processor, rather than scattered around in various parts of PDU processing.

This removes many dependencies and assumptions about the order of arrival of PDUs and makes the protocol handling more robust and accurate per the blue book spec.

Notably, this fixes a number of important concerns:

• Out of order MD arrival is no longer an exception, the state machine is identical regardless if MD comes first or not.
• Out of order arrival is handled in class 1 (PDUs can arrive in any order)
• Logic for file retention is consistently applied in both success and error cases - always goes through this part of the state machine.
• Allows transferring a zero byte file (previously would error out because of the way the "read" call was reached).
• Files are no longer updated in place. This means an existing file will not be clobbered until a complete new file is received.
• Bit errors in an FD PDU header simply drop the PDU (it will be NAKed) rather than aborting the entire transaction.
• ACK/NAK counting is accurately implemented in class 2.
• The CC response in EOF/FIN packets will be accurate when the ACK limit is reached.
• Tx-side tick processing simplified, there is now just one tick processor, all the special "cont" and "early_exit" flags removed.

The bottom line: all transactions will proceed through the state machine in the exact same way, whether class 1 or class 2, with transitions all handled in the same place and in a consistent manner, going through all the same checks and validations on every transition in every transaction, regardless of PDU arrival order. This greatly improves adherence to the CFDP protocol spec and ability to deal with imperfect PDU delivery.

Bonus item: The "closure request" feature of class 1 transfers now is supported and works in the engine. (Only missing part is a way to request it when initiating a Tx transaction)

Fixes #481

Testing performed
Execute various file transfers, class 1 and class 2.
Use python script to force protocol anomalies such as out-of-order PDUs and PDUs with errors, confirm correct handling.

Expected behavior changes
Listed above

System(s) tested on
Linux

Contributor Info - All information REQUIRED for consideration of pull request
Joseph Hickey, Vantage Systems, Inc.

[github-advanced-security]

CodeQL found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.