Some remote api (for instance the scheduler one) does not declare any
extra_encoders in the negotiate decorator. This was raising an error
when querying any declared endpoint.

I forgot to change the async server in
08289ce5.

iso8601 is more strict, and less inefficient.

They will allow swh-storage to override them to serialize and
deserialize swh-model objects.

This will allow swh-storage to pass model objects.

This breaks the current protocol, but deduplicates the code, and will allow
for the addition of extra custom encoders/decoders by callers of this module.

Pros:
* Language-independent
* Works with unpickleable exceptions (eg. cimpl.KafkaException, and
  RLock objects in exceptions raised by tenacity)

Con:
* Shows a less nice error on the client side (if the client is in
  Python), unless the error is whitelisted in `reraise_exceptions`.

 Fixed consistency problems

Optional params are now Optional

Data is now bytes

Rremoving them allows testing the function's signature
matches the existing signature of a specification and type checking.

Moreover, they should not be used by users of the class, so there is no
reason to have them appear in the documentation (generated from
the signature).

This allows initializing Sentry with the environment the process
is running in (ie. "production" or "staging"), which allows filtering
in the web UI to only show issues happening in one of the environments.

https://docs.sentry.io/enriching-error-data/environments/?platform=python

Related to T2188

This enables tracking of issues by release/package version in sentry
(https://docs.sentry.io/workflow/releases/).

Also deduplicates code used to call sentry_sdk.init by making all our
init code go through the same function swh.core.sentry.init_sentry."

Related to T2110

It was systematically named that way in code.

Having tox call tox is a bit weird, but the alternatives are:
 - duplication of the test command, which is a bit sad
 - replacing all the positive factors with conjunction of negative factors, e.g.
   `core` -> `!db-!server`, which makes the logic painful to follow

One day, tox will have environment aliases which will make this horror moot :-)

(Idea pulled from a comment in https://github.com/tox-dev/tox/issues/427)

The current tests depend on order of execution: if the main tests run before the
db tests, the addition of extra subcommands makes them fail.

Makes it easier to save/restore it in test fixtures.

This requires some more stub-missing entries declared in mypy.ini to pass OK.

swh.storage raises TypeErrors when type checking of incoming objects fails. We
need to make sure that these go through the RPC layer unchanged.

When run under a systemd service, the default stop command is to kill the
process; By default, Python doesn't trap sigterm and terminates the process
immediately, so even finally blocks don't run.

This traps SIGTERM and SIGINT (^C) signals and raises an exception, allowing the
process to exit in an orderly fashion.

for improved readability and control flow understandability

to that end, refactor HTTP status checking logic so that it is reused in
various places