Server-side CHUNKING and BINARYMIME support

[foxcpp]

See #81. This issue is not closed since it also references client support.

[codecov]

Codecov Report

Merging #104 into master will increase coverage by 2.12%.
The diff coverage is 70.76%.

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
+ Coverage   62.52%   64.64%   +2.12%     
==========================================
  Files           8        9       +1     
  Lines         982     1140     +158     
==========================================
+ Hits          614      737     +123     
- Misses        275      302      +27     
- Partials       93      101       +8     

Impacted Files	Coverage Δ
smtp.go	100.00% <ø> (ø)
conn.go	59.36% <57.62%> (+0.01%)	⬆️
chunk_reader.go	100.00% <100.00%> (ø)
server.go	62.24% <100.00%> (+6.12%)	⬆️
parse.go	57.89% <0.00%> (+5.26%)	⬆️

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[emersion]

Instead of this chunkReader approach, can't we read data directly from handleBdat and forward it to the backend via an io.Pipe?

pr, pw := io.Pipe()
go c.Session().Data(pr)
lr := io.LimitedReader{R: c.text.R, N: size}
io.Copy(pw, lr)

[foxcpp]

Instead of this chunkReader approach, can't we read data directly from handleBdat and forward it to the backend via an io.Pipe?

I wanted to preserve ability for backend to return errors without consuming the whole message but I will look if this is easily possible with io.Pipe.

[emersion]

I wanted to preserve ability for backend to return errors without consuming the whole message but I will look if this is easily possible with io.Pipe.

io.PipeReader.CloseWithError could help.