Add per-domain SMTP routing (knadh#2953)

Currently, you can add multiple SMTP servers, but they will be
round-robin'd. It is useful to be able to pick an SMTP server based on the `from`
of the campaign. For example, Mailgun will DKIM sign based on the domain of the
authenticating user, so if you send campains from more than one domain,
you need to have different SMTP servers setup for each domain.

This adds an optional list of domains that each SMTP server should be
used with. If multiple servers specify the domain in question, we will pick one
at random, just as the logic today does.

If no domains are specified, or non match, then we fall back
to the existing behavior of picking one at random.

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Signed-off-by: Phil Dibowitz <phil@ipom.com>
Co-authored-by: Kailash Nadh <kailash@nadh.in>