Track maximum runtime of currently running jobs

[Envek]

This is alternative implementation of #13 without querying Redis.

The goal:

useful metrics to keep track of the current running job runtime (see the "busy" section in the sidekiq admin panel).
This is mostly for alerting to determine time-consuming jobs

However, in implementation in #13 each Sidekiq worker executes query to Redis on each collect block execution, retrieves info about all executing jobs in all Sidekiq processes and single process reports not only its own stats, but also stats for every other worker. On large Sidekiq installations (with dozens processes executing hundreds of jobs at a time) it may be painful.

In this pull request, every worker process keeps track and reports runtime duration only for its own jobs in memory and doesn't any additional requests to Redis.

[Envek]

@dsalahutdinov, released in 0.8.0. Enjoy!

[dsalahutdinov]

@Envek thx,
Btw, it might be useful for Amplifr, @dreikanter please take a look

[dreikanter]

@dsalahutdinov Cool, thank you! Will definitely use it. Thanks @Envek!