Merge branch '8.x-3.x' into MADS-56-queue-settings-per-mapping

* 8.x-3.x:
  Couple more adjustments for standalone push queue processor; tested and verified this time
  Moves queue_processor into subordinate method
  Expose standalone push queue global setting, and standalone push queue per mapping.
  - Adds example documentation for using RestClient for an Apex endpoint - Update RestClientInterface with new public method
  - Don't pull mappings which don't do pull. - Add exception handling inside enqueue method
  Change apiCall() to allow RestClient to be used for custom endpoints

# Conflicts:
#	modules/salesforce_mapping/src/Form/SalesforceMappingFormCrudBase.php
#	modules/salesforce_pull/src/QueueHandler.php
#	modules/salesforce_push/src/PushQueue.php
#	src/Form/SettingsForm.php