Grjones use setnx for locking

[cameronmaske]

#60 but add's extra documentation and countdown back to AlreadyQueued exception.

[coveralls]

Pull Request Test Coverage Report for Build 197

• 9 of 11 (81.82%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-1.9%) to 96.753%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
celery_once/backends/redis.py	8	10	80.0%

Files with Coverage Reduction	New Missed Lines	%
celery_once/helpers.py	1	97.5%

Totals
Change from base Build 189:	-1.9%
Covered Lines:	149
Relevant Lines:	154

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💛 - Coveralls

[cameronmaske]

Steps to remove existing keys for 1.0 -> 2.0 migration.
NOTE: These are untested, use at your own risk.

This will list all celery-once key's (starting with the prefix qo_) then delete them...

redis-cli KEYS "qo_*" | xargs redis-cli DEL