Eliminate usage of Zend_Mail from Magento 2 Open Source

[vkublytskyi]

References count: 4
Used in EE/B2B: 0

Acceptance Criteria:

1. Usage of Zend_Mail removed from Magento 2 Open Source
   • all references to Zend_Mail are removed from Magento 2 Open Source
   • added static a test that verifies that Zend_Mail is not used
   • Travis builds are green on PHP 7.1 environment
   • all backward incompatible changes are documented (if any)
   • if possible ZF1 component replaced by embedded PHP functionality or PSR interface otherwise migrated to ZF2
   • if ZF1 component replaced by ZF2 component:
     • there is an explanation why it is required to use external dependency
     • if usage of old ZF1 component was exposed in M2 interface/class signatures it is removed for new code
     • there is a list of PHP 7.2 compatibility issues of new ZF2 component
       OR
2. There is an explanation why it is impossible to delete this dependency now:
   • described scope of work that is necessary to totally remove usage of Zend_Mail
   • described what backward incompatible changes will be caused by dependency removal
   • listed what other ZF1 components are required for Zend_Mail to support M2
     (if only limited functionality of Zend_Mail is used by M2
     then not all dependencies may be required and some part of Zend_Mail
     still may be removed if has incompatibility with PHP 7.2)
   • listed PHP 7.2 compatibility issues for Zend_Mail