Adds additional rake task to clear jobs from a specific queue

[azeezahmed]

Currently, the 'jobs:clear' task clears all the records from the delayed_job table however, there will be scenarios where jobs only from a specific queue need to be cleared.

ex: restarting (post-maintenance) you would only want to clear WebSocket jobs (news feed) but not subscription charges job (monthly billing).

this pull request adds an additional rake task that accepts a parameter(queue_name) and clears the jobs from the queue.

[coveralls]

Coverage remained the same at 90.934% when pulling d4f404d on azeezahmed:master into bd07783 on collectiveidea:master.

[kou]

I want this feature too but this implementation isn't suitable.
Because it works only with Active Record backend.

We need to add a new backend API such as Delayed::Job.delete_all(queues: nil) and Delayed::Job.delete_queues(queues) to make this feature portable.

And I think that we don't need to add a new task. We can reuse jobs:clear task like the following:

diff --git a/lib/delayed/tasks.rb b/lib/delayed/tasks.rb
index 409ba48..edacb18 100644
--- a/lib/delayed/tasks.rb
+++ b/lib/delayed/tasks.rb
@@ -1,7 +1,12 @@
 namespace :jobs do
   desc 'Clear the delayed_job queue.'
   task :clear => :environment do
-    Delayed::Job.delete_all
+    queues = (ENV['QUEUES'] || ENV['QUEUE'] || '').split(',')
+    if queues.present?
+      Delayed::Job.delete_queue(queues)
+    else
+      Delayed::Job.delete_all
+    end
   end
 
   desc 'Start a delayed_job worker.'