feat(worker): allow number of threads to be configured

This allows for more threads to be run. Care needs to be taken to ensure that database connection pool size is appropriate for this.

app/lib/worker/process.rb

@@ -45,7 +45,9 @@ module Worker
     ].freeze
 
     # @param [Integer] thread_count The number of worker threads to run in this process
-    def initialize(thread_count: 2, work_sleep_time: 5, task_sleep_time: 60)
+    def initialize(thread_count: Postal::Config.worker.threads,
+                   work_sleep_time: 5,
+                   task_sleep_time: 60)
       @thread_count = thread_count
       @exit_pipe_read, @exit_pipe_write = IO.pipe
       @work_sleep_time = work_sleep_time