Fix tedge-agent stuck on too many pending operations

[didier-wenzek]

Proposed changes

Introduce unbounded receivers. Given a cycle of actors (say A sending messages to B, B to C and C to A), at least one of these actors must use such an unbounded receiver.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

#2639

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
387	0	3	387	100	53m28.617s

[codecov]

Codecov Report

Attention: 67 lines in your changes are missing coverage. Please review.

Comparison is base (6bb5198) 75.8% compared to head (862357a) 75.9%.
Report is 83 commits behind head on main.

Additional details and impacted files

Files	Coverage Δ
...tedge_agent/src/tedge_operation_converter/actor.rs	47.1% <ø> (ø)
...dge_agent/src/tedge_operation_converter/builder.rs	90.2% <100.0%> (ø)
crates/core/tedge_api/src/message_log.rs	84.0% <100.0%> (ø)
crates/extensions/c8y_mapper_ext/src/converter.rs	81.1% <100.0%> (+0.1%)	⬆️
...s/c8y_mapper_ext/src/operations/firmware_update.rs	88.6% <80.0%> (+0.4%)	⬆️
...es/extensions/c8y_mapper_ext/src/operations/mod.rs	91.5% <94.8%> (+4.8%)	⬆️
crates/core/tedge_api/src/entity_store.rs	92.9% <0.0%> (ø)
...nsions/c8y_mapper_ext/src/operations/log_upload.rs	88.8% <89.1%> (+0.4%)	⬆️
crates/extensions/tedge_mqtt_ext/src/lib.rs	64.7% <16.6%> (-1.4%)	⬇️
crates/core/tedge_actors/src/channels.rs	61.7% <41.6%> (-2.4%)	⬇️
... and 2 more

... and 5 files with indirect coverage changes

[didier-wenzek]

An alternative to fix the issue could be to use two receivers.

• One receiver to receive progress feedback from the software actor
• Another to collect requests from MQTT

Using a biased select, one can then give the priority to the feedback messages.

[rina23q]

LGTM