Further update and refactor D8 in prep for alpha release by evanjenkins, ironsizide, aaronbauman:

- Issue #2809557 by evanjenkins - update API version
- Compatibility updates for D8.3.x
- update hook_help for new APIs
- Removes use of String class - see https://www.drupal.org/node/2457593
- Various classes/interfaces renamed to support PHP 7
- Fleshes out mapping config helper methods
-- salesforce_mapping_load and salesforce_mapping_load_multiple working, not as static methods on the MappedObject class, but using D8 constructs to get them working for refining other methods.
-MappedObjectForm::submitPull greatly simplified, pending review
- Makes numerous changes to cron pull code
-- Injects soap into cron helper function so object is created once instead of three times.
-- Comments out record type code in pull item creation code until the logic is straightened out.
-- Uses correct SelectorQuery class.
-- Moves the cron pule queue processor to QueueWorker plugins (so it actually runs now).
-- QueueWorker plugin stills needs some logic corrections (bitwise vs string compare operators).
-- base module RestClient service updated to use correct SelectQuery class.
-- .gitgnore updated to prevent .tags file inclusion.
- Adds stubs for mock REST and SOAP servers
- remove "Salesforce" from class names in salesforce-namespaced classes
- Fix empty install config that were throwing exceptions on install
- Convert config items to state items
- Fixes basic cron pull functionality
-- Renames Pull QueueWorker classes to remove ‘Salesforce’.
-- Fixes pull logic triggers (array keys vs bitwise logic).
-- Modifies MappedObject::pull method to be cron and manual capable, used in cron QueueWorker.
-- Removed entity wrapper code (baked in in D8!).
-- Fixes salesforce_mapped_object_load_by_sfid to use D8 load methods.
-- Added passed object support to MappedObject::pull to avoid duplicate object loading.
- Fix field mapping form
- Fix schema issues on mapping properties form
- Overhaul "upsert key" logic:
-- Promote to first-level property of mapping object.
-- Change field mapping input into a select field.
-- Eliminate from entity all extraneous logic for juggling old approach
-- Issue #1992260 - Make sure RestClient::upsertObject always returns a Salesforce ID
- make push / pull buttons work on mapped object view
- Tweak properties and fields forms for previous API changes
- Convert array syntax. Thanks https://github.com/thomasbachem/php-short-array-syntax-converter
- Issue #2642094 - Add pull trigger date field to mapping
- Add pull trigger date to PullBase.php