smtp 8.x-1.0-rc4

This (hopefully) is the last bugfix before 1.0 for SMTP module.

Bug changes since 8.x-1.0-rc3:

• Issue #3150369 by japerry: Ensure caches are cleared when updating connection tester service.
• Issue #3150369 by mmbk, TR: Typo in service name 'smtp.contection_tester'
• Issue #2925878 by alexrayu, zerbash, holist, wundo, dww, klevyke, Chris Matthews: SMTPMailSystem does not respect $from and $from_name values
• Issue #3142849 by sunlix, drupalfan79, JoCowood: Invalid mail address causes spool to stop
• Issue #2909678 by jlicht, espurnes: Content-Transfer-Encoding header within multipart messages not parsed properly
• Issue #3153720 by TR: ConnectionTesterTest isn't running because it has the wrong namespace
• Issue #3019422 by TR: Immutable Configuration Error
• Issue #3155928 by jdickmann: Timeout field has no validation and can be set blank
• Issue #3156081 by tsplash: PHPMailer timeout set as empty when smtp_timeout is empty
• Issue #3156075 by malcomio: Update hooks removed