Backend-ai-manager

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

**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).

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

**CHANGES**

- When launching a new kernel and accessing to an existing kernel, it scans
only "currently alive" instances by checking shadow keys that automatically
expires. This makes the Sorna service sustainable with abrupt agent failures.

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

**CHANGES**

- When launching a new kernel, it now chooses the least loaded agent instead of
the first-found agent with free kernel slots.