Remove `Message.without_bcc` ctor since we can now get GMime to exclude
BCC headers when serialising, avoiding the overhead of taking a complete
copy of the message just to strip BCCs.
Rename `get_network_buffer` to `get_rfc822_buffer` to be a bit more
explicit in that's the way to get an actual RFC822 formatted message.
Replace book args with flags, and take a protocol-specific approach
rather than feature-specific, because in the end you're either going to
want all the SMTP formatting quirks or none of them.
Remove custom dot-stuffing support from Smtp.ClientConnection, since it
is redundant.
Make constructors use the Message.from_gmime where possible. Merge that
and stock_from_gmime so it's clear the stocking only happens in the
ctor. Update stocking to use high-level GMime APIs where available to
avoid re-parsing header values.
