Stop using SETNX for locking

[thesecretmaster]

According to the documentation, you shouldn't use SETNX for locking and instead use the Redlock algorithm as described here. Since there's already a redlock-rb gem, integrating it shouldn't be too hard.

[nateware]

Sounds interesting! Would definitely accept a patch for this. CC @tmsrjs

[allisonphillips]

@thesecretmaster Did you intend to link to this project? The redlock-rb project linked above hasn't had a commit since 2015.