(PUP-2987) Restructure Windows service code

 - Move runinterval parsing and execution of the puppet agent to a new
   thread that maintains a busyloop until either:
    - the service is no longer running
    - an exception has occurred with parsing the runinterval
 - Rather than performing a wakeup against the main thread, simply kill
   this new thread on service_stop
 - Main service code will wait until the busyloop thread dies, then will
   exit
 - Note that this change is ultimately not responsible for the service
   hangs experienced in the service.  Additional changes have been made
   to the win32-service gem to ensure events being waited on with
   WaitForSingleObject are always signaled properly, even in failure
   cases.  Please see merged PR for more info:
   https://github.com/djberg96/win32-service/pull/21
   https://github.com/puppetlabs/puppet-win32-ruby/pull/51