Allow specifying a custom LeaseManager in Worker.Builder with tests

[felipegasparini]

Continuing the work started on PR:
#255

(sorry for creating a new PR)

Added tests as requested.

[pfifer]

Thanks for providing this. I'm looking at the changes, but don't have an ETA right now.

[felipegasparini]

Ok! Let me know if you need something else.
My use case is similar to the others one that requested this change: Be able to use another implementation of LeaseManager (baked by another datastore).

If you have another solution to solve this problem that makes more sense for the project, let me know.

[pfifer]

I just need everyone to please confirm that we can use, modify, copy, and redistribute this contribution.

Thanks

[felipegasparini]

Sure! How can I do that?
Do you have a contributors license agreement or something similar?

[pfifer]

No CLA is required, just need you to agree for us to distribute this. Did you based the changes off of @johnsonwj changes? If that's the case I need them to also agree to allow us to distribute the changes.

[felipegasparini]

From my side there is no problem at all.
And yes, I actually just continued @johnsonwj contribution. So let's wait for him.
Thanks!

[johnsonwj]

Apologies for not seeing this earlier! You have my permission to use/modify/copy/redistribute my contribution!

[felipegasparini]

You also have my permission to use/modify/copy/redistribute my contribution.

[felipegasparini]

Thanks, @pfifer !