Fix crash when trying to clear the worker list.

Fixes #1023.
Pass an array to srem, but only if non-empty.
Use !empty? instead of any?
Merge pull request #1024 from kek/master

Fix crash when trying to clear the worker list
Add custom retry delay
Log retry delay failures and fallback to the default
fixup
Let them know it's being retried using the default
fixup
Refactor into a class
space fix
Revert "Refactor into a class"

This reverts commit 9583f090a80236db36c2860a80fda092dd5f453c.
Merge pull request #1027 from jmazzi/feature/custom_retry

Add custom retry delay
Fix heisenbug with tests
Refactor sidekiq_retry_in impl
Prefer background jobs to message processing in future terminology
Allow retries_exhausted to be defined in a block

sidekiq_retries_exhausted { |*args| }
Deprecate Worker#retries_exhausted in favor of the
`sidekiq_retries_exhausted` callback
Add link to Error-Handling wiki page
Pass the entire message to sidekiq_retries_exhausted