make get_redis_connection to accept dict-like 'opts' argument

fix misleading warning for non-copr library consumers
add `contextlib.nullcontext` function as EL8 compat
limit stdout/stderr of ssh.run_expensive() commands
use 'copr-common/<version>' as http user agent identifier
changes needed to allow user SSH to builders

rename GroupWorkerLimit to HashWorkerLimit
explicitly build-depend on python-six

move chroot_to_branch from frontend to copr-common
redis authentication support added

Specfile compatibility with OpenEuler

Periodically dump the priority queue to a JSON file
Use SPDX license

move to GitHub home page
logging shouldn't affect stdout
move dispatcher and background workers to copr-common
scripts should log also timestamps etc when logging into file
move setup_script_logger to copr-common

Add background_worker.py from backend
Add get_redis_connection function
Add Dispatcher, WorkerManager, and QueueTask classes
Add WorkerLimit, PredicateWorkerLimit, and GroupWorkerLimit classes

Add setup_script_logger function

Allow SafeRequest's timeout to be specified