Sorna-manager

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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.)

------------------

**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.

------------------

**FIXES**

- Make sure all errorneous responses to contain RFC 7807-style JSON-formatted
error messages using aiohttp middleware.

------------------

**FIXES**

- Assume date headers in HTTP request headers without timezone offsets
as UTC instead of showing internal server error.

------------------

**NEW**

- Deprecated legacy ZMQ interface. The code is still there, but should
not be used.

- Refined keypair/usage database schema.

- Implemented the streaming-mode API: web terminal!

- Restarting the kernel in the middle of web termainl session are transparently
handled -- user's browser-side websocket connections are preserved.

- The codebase now requires Python 3.6.0 or higher.

- Internally it adopted a simple event bus to handle asynchronous docker events
such as abnormal termination of kernels. Now most interactions with docker
are truly asynchronous.

------------------

**FIXES**

- Legacy ZMQ interface: Revived a missing language parameter in legacy
client-side session token generation.
This has broken CodeOnWeb's PRACTICE page.

- Gateway: Increased timeouts when interacting with agents.
In particular, code execution timeouts must be longer than kernel execution
timeouts.

- Gateway: Added a missing transaction context during authorization.
This has caused "another operation in progress" errors with concurrent API
requests within a very short period of time (under a few tens of msec).