Backport FileDownlink timeout removal to v3.6

[nathancheek]

F´ Version	v3.6.4
Affected Component	Svc.FileDownlink

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Problem Description

A buffer ownership bug was fixed in v4.2.0, see #4555. It would be nice to backport this fix to v3.6.x.

This bug was found by implementing and placing a ThrottledBufferRepeater downstream of a FileDownlink component, and setting the throttle timing high enough that it triggered a FileDownlink timeout. This brought to light a bug in bufferReturn_handler(), where FileDownlink doesn't properly handle receiving buffers while in COOLDOWN mode.

The fix in #4555 should keep us from hitting the COOLDOWN bug in normal conditions, since FileDownlink won't enter COOLDOWN mode until the buffer has been returned. However, I would also like to update bufferReturn_handler() so it knows how to handle returned buffers while in COOLDOWN mode. Here's how bufferReturn_handler() currently handles returned buffers based on the component's mode:

• IDLE: Does nothing
• CANCEL: Sends appropriate next packet
• WAIT: Sends appropriate next packet
• DOWNLINK: asserts, but this is okay since the DOWNLINK mode is a temporary mode only used while inside of a bufferReturn_handler() call, so we should never be in DOWNLINK mode at the start of a bufferReturn_handler() call
• COOLDOWN: asserts, not ideal! But would require the downstream component to return a packet at an unexpected time.

I think returned buffers should be handled the same way in both IDLE mode and COOLDOWN mode. Seeing as how bufferReturn_handler() already has a non-asserting offramp for buffers returned during IDLE mode, I propose we do the same for COOLDOWN mode. I believe this COOLDOWN fix would also be useful for F Prime v4... if there is any concurrence I can open an issue and PR to pull that change into dev.

I will open a PR shortly with the #4555 fix cherry-picked on top of v3.6.4, along with a fix so bufferReturn_handler() handles COOLDOWN mode.

How to Reproduce

1. Place a component downstream of FileDownlink that takes longer than FileDownlink's m_timeout, but shorter than m_timeout + m_cooldown, to return.
2. Once the timeout occurs, FileDownlink will assert in bufferReturn_handler() when it receives the returned buffer while in COOLDOWN mode.

Expected Behavior

FileDownlink should not assert when it receives a returned buffer while in COOLDOWN mode.

[nathancheek]

This was merged via #5288.