v7.7.0 improved connection pools and cluster config

• #430 Enhancement: auto-reconnect (if needed) when reclaiming a Transport connection from the SMTP connection pool (to avoid needless errors)
• #383 Feature: be able to set defaults and overrides on the Mailer level, rather than email or global level

This moves the conversion of Email to MimeMessage to after a Transport instance has been selected (in case of a cluster of SMTP servers),
so we can apply defaults/overrides on the Mailer level, meaning you can configure 'global' values for individual SMTP servers