Define Config class inside SidekiqUniqueJobs module

[Slike9]

Config class used to be defined in Concurrent::MutableStruct which is external, i.e. defined in other library. I guess, it's not good practice to define your code inside a foreign module.
By the way, the pr also fixes #340.

[mhenrixon]

Thank you @Slike9, the code you suggests I believe will only work for concurrent ruby versions greater than 1.1.1 so we need to enforce that then or provide a fallback to allow both older and newer versions of concurrent ruby to be used. Very please to see this update in concurrent ruby!! I really did not like the way they wanted me to define the config class originally.

[Slike9]

@mhenrixon sidekiq-unique-jobs requires

spec.add_dependency 'concurrent-ruby', '~> 1.0', '>= 1.0.5'

concurrent-ruby v1.0.0 already supports the api https://github.com/ruby-concurrency/concurrent-ruby/blob/v1.0.0/lib/concurrent/synchronization/abstract_struct.rb#L139. So, the fix will work for all now supported versions of concurrent-ruby. I've run the tests against concurrent-ruby '1.0.5' and they've passed.