add locker for distributed scheduling

[JohnRoesler]

What does this do?

adds the WithDistributedLocker() method to the scheduler that takes in a Locker allowing for locking jobs when multiple schedulers are running.

The initial implementation is with redis. However, additional implementations may be added here via PR or you may simply implement your own Locker.

Which issue(s) does this PR fix/relate to?

#181
#180

List any changes that modify/break current functionality

Have you included tests for your changes?

yes

testcontainers will soon be officially supporting redis (it's not released, but is available on main) -> https://golang.testcontainers.org/modules/redis/

Super awesome project!

Did you document any new/modified functionality?

• Updated example_test.go
• Updated README.md

Notes

This is called out in the function docs for the new method, but I will post it here also:

// WithDistributedLocker prevents the same job from being run more than once
// when multiple schedulers are trying to schedule the same job.
//
// One strategy to reduce splay in the job execution times when using
// intervals (e.g. 1s, 1m, 1h), on each scheduler instance, is to use
// StartAt with time.Now().Round(interval) to start the job at the
// next interval boundary.
//
// Another strategy is to use the Cron or CronWithSeconds methods as they
// use the same behavior described above using StartAt.
//
// NOTE - the Locker will NOT lock jobs using any of the limiting functions:
// SingletonMode, SingletonModeAll or SetMaxConcurrentJobs