0.27rc1

WARNING: The changes are backward-compatible (in theory). But the changes are also massive, so things can break unexpectedly (in practice). Test this upgrade carefully.

New features:

• @kopf.daemon for background resource-accompanying tasks/threads. #330
• @kopf.timer for regular and/or delayed activities & checks. #330
• kopf.PRESENT for labels/annotations filters (instead of misleading None). #327
• kopf.ABSENT for labels/annotations filters. #327
• Arbitrary callbacks for labels/annotations filters. #328
• A proper official documented way to configure operators; plus docs. #336 #337
• Operators can be configured to use their own names for persistence, allowing multiple non-colliding operators for the same resources. #331

Improvements:

• Internals instant functions (lifecycles, when-filters) do not use thread pools anymore. #326
• Documentation is restructured for separate pages for all aspects of handling (kwargs, filters, etc). #325
• Bodies & patches now have magical properties for metadata, spec, status, with "live view" into data. #327
• Labels and annotations are passed as additional kwargs (similar to body/spec/meta/status). #327
• Configurable client-side timeouts for watch requests. #322 #336
• Configurable storages for handling state & diff-bases. #331

Fixes:

• Threads from the sync handlers executors are not leaked (not left orphaned) at exit. #326
• Handlers' state is now persisted in annotations, not in status: for custom & built-in resources. This solves the issues with built-ins not handled properly in K8s 1.16+. #331

Internal changes:

• Compatibility with Kubernetes 1.18 in CI/CD. #335