Purge all deprecated methods and classes for v1.x

[nolar]

Drop all features that were declared deprecated a long time ago. The exact list is below — with the supposed replacements.

• cooldown= option of decorators → backoff=.
• event kwarg in @kopf.on.event → reason.
• cause kwarg in handlers → use all kwargs directly.
• kopf.login() sync function → async activity internally.
• kopf.config module → settings kwarg of @on.startup() handlers.
• get_pykube_cfg() for patching → @on.login() handlers.
• HandlerFatalError → PermanentError
• HandlerRetryError → TemporaryError
• kopf.create_tasks() (sync) → kopf.spawn_tasks() (async).
• kopf.events.[event|info|warn|exception]() → the same directly in kopf module.
• None for labels=/annotations= filters → kopf.PRESENT (None now raises exceptions).
• Positional field in @on.field('group', 'v1', 'plural', 'field') → kwarg field='field' instead.
• Unversioned keys in storages → generate v1/v2 keys explicitly.
• Storages with unprefixed annotations → prefix is now mandatory; defaults to kopf.zalando.org/….
• @kopf.on.this() for sub-handlers → @kopf.subhandler() (for readability)

Most of these legacies are extremely old and it is highly unlikely that they were used in any existing operators. In any case, the DeprecationWarnings were issued for all these features, so there should be enough time to switch.

THIS IS A BREAKING CHANGE. => Major version increase is required.

Unlike #625 (dropping the legacy registries), these features are not difficult to maintain in backwards-compatible mode. However, since #625 is now required for the reactor's refactoring, and therefore the major version has to be increased — it seems like a good idea to also drop all other legacies from the code to make it cleaner.

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PREVIOUS DESCRIPTION:

This PR is a preview of what will be dropped from the next major release (1.0) when it happens.

There are no specific dates or estimations or even criteria of readiness for 1.0. But the preparations can be performed now.

All deprecated classes and methods and functions do issue warnings now (DeprecationWarning), so the operator developers should pay attention to them and switch to the recommended constructs.

This branch is regularly synced with the recent master — to keep the release-1.0-look-ahead code always up to date and tested.

Replaces #316 zalando-incubator/kopf#316

[lgtm-com]

This pull request fixes 1 alert when merging 421e187 into caaf399 - view on LGTM.com

fixed alerts:

• 1 for Unused import

[lgtm-com]

This pull request introduces 5 alerts and fixes 1 when merging a3c1f66 into d712db2 - view on LGTM.com

new alerts:

• 5 for Unused import

fixed alerts:

• 1 for Unused import

[lgtm-com]

This pull request introduces 5 alerts when merging 0adabf4 into 7567d71 - view on LGTM.com

new alerts:

• 5 for Unused import

[lgtm-com]

This pull request introduces 1 alert when merging 72a3276 into 878a5af - view on LGTM.com

new alerts:

• 1 for Unused import

[lgtm-com]

This pull request fixes 2 alerts when merging 3c5aef2 into 878a5af - view on LGTM.com

fixed alerts:

• 2 for Unused import