Implement an alternative to DisableConcurrentExecution attribute

The implementation is crappy for long-running jobs, because it requires a long-running distributed lock. And it's totally unclear what timeout to use, because it should cover the maximum possible duration of a background job. The following problems arise, when using high values for timeouts (consider 4 hours, for example):

1. Any worker that fetched a job protected by this attribute will wait, and will wait for 4 hours in our example. Have multiple attributes – the whole processing could be stopped.
2. For very high values, like 24 hours, resource governors may simply release the distributed lock, and we'll get 2 or more concurrent executions depending on the job duration.

If we are using low values, we could end up with permanent `DistributedLockTimeoutException` or `TimeoutException` depending on the version. There's the [`MutexAttribute`](https://gist.github.com/odinserj/4a3bf40606c4da9183588a5a325dfb99) that was implemented as a gist, and it is using short distributed locks, but its implementation could be greatly simplified by using a single hash value instead of sets.

So if someone wants to deep dive into Hangfire internals and extensibility points, this is a good entry point!

Start with the linked MutexAttribute gist and Hangfire's attribute, distributed-lock, and extensibility entry points. Compare the gist's short-lock approach with the current DisableConcurrentExecution behavior, including the single-hash simplification. Done means a defined replacement that avoids long-held locks while preserving mutual exclusion for long-running jobs.

Written by the indexing model from the issue text.