Backend-ai-manager

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- Fix sorna-common requirement version.

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**IMPROVEMENTS**

- Handle v1/v2 API requests separately.
Now it preserves old "aggregated" stdout/stderr/media outputs for v1
but uses the new streaming outputs for v2.
(v1 API users can use streaming as well, but they will loose the
ordering information of individual lines of the console output.)

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**FIXES**

- Fix task pending error during shutdown due to missing await for redis
monitoring task after cancelled.

- Fix wrong active instance count in Datadog stats due to missing checks for
shadow in ``InstanceRegistry.enumerate_instances()``.

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**FIXES**

- Prevent potential CPU-hogging infinite loop during Datadog stats updates.

**IMPROVEMENTS**

- Add statistics reporting via Datadog. (optional feature)

- Improve exception handling and reporting, particularly for agent-sid errors.

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**FIXES**

- It now copes with API requests without bodies at all: use an empty string to
generate signatures.

- Enabled authorization checks to stream-mode APIs, which has been disabled
for debugging and tests.
(Though the probability of exposing kernels to other users was very low
due to randomly generated kernel IDs.)

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**FIXES**

- Stabilized sporadic restarts/disconnects of agent instances, and keep the
concurrency usage consistent.

- Increased the minimum size of aioredis connection pools to avoid rare
deadlocks due to pool exhaustion.