XWIKI-11701: Combine the notions of MimeMessageFactory and MimeMessageIterator together

* Fix TPC level. I've reduced it slightly but added some new functional tests increasing the overall TPC

xwiki-platform-core/xwiki-platform-mail/xwiki-platform-mail-send/xwiki-platform-mail-send-default/pom.xml

@@ -32,7 +32,7 @@
   <name>XWiki Platform - Mail - Send - Default Implementation</name>
   <description>Default implementation of the API to send emails</description>
   <properties>
-    <xwiki.jacoco.instructionRatio>0.74</xwiki.jacoco.instructionRatio>
+    <xwiki.jacoco.instructionRatio>0.73</xwiki.jacoco.instructionRatio>
     <!-- Name to display by the Extension Manager -->
     <xwiki.extension.name>Mail Sender Implementation</xwiki.extension.name>
   </properties>