Implement SMTP connection / client reuse

[dannykopping]

Currently our SMTP clients connects to the smarthost for every email dispatched (as Alertmanager does).

We should look into pooling connections or clients to reduce overhead and possible rate-limiting issues.

See https://github.com/knadh/smtppool for inspiration.

[stirby]

We can leave this "parked" until customers raise rate-limiting issues.