Convert watchStuckJobs implementation to use SCAN instead of KEYS

[taywrobel]

Adjusts the implementation of the watchStuckJobs LUA script to use a paginated SCAN of the keyspace, rather than the current KEYS approach.

This has a significant impact in large or shared redis deployments where the keyspace can be unbounded in size. KEYS will block all execution as it runs, which can take several seconds or longer whereas the SCAN approach permits other operations to be run in between each page of results.

[vjustov]

👍 when could this be reviewd or merged?

[snypelife]

This is a pretty gnarly issue. Any chance this PR could get some traction?

[niclic]

Last commit to master in January 2018? Don't hold your breath. @snypelife

[snypelife]

womp womp 👎

[rahulroy9202]

bump

[igauravsehrawat]

We had to cherry pick this code with modification to kue.prototype.watchStuckJobsMod