More mail API churn... reversing some previous API decisions.
I've made some key API changes to EMailSession on the account-mgmt
branch which should allow for this, and will hopefully also benefit
the "email-factory" branch.
EMailBackend barely needs to exist anymore, except as the owner of
EMailSession.
For several low-level functions, we replace its EMailBackend parameter
with EMailSession and EAlertSink parameters; the latter so it can still
pass user alerts up the chain.
a9cfed5938