mime: do not perform more than one read in a row

Input buffer filling may delay the data sending if data reads are slow.
To overcome this problem, file and callback data reads do not accumulate
in buffer anymore. All other data (memory data and mime framing) are
considered as fast and still concatenated in buffer.
As this may highly impact performance in terms of data overhead, an early
end of part data check is added to spare a read call.
When encoding a part's data, an encoder may require more bytes than made
available by a single read. In this case, the above rule does not apply
and reads are performed until the encoder is able to deliver some data.

Tests 643, 644, 645, 650 and 654 have been adapted to the output data
changes, with test data size reduced to avoid the boredom of long lists of
1-byte chunks in verification data.
New test 667 checks mimepost using single-byte read callback with encoder.
New test 668 checks the end of part data early detection.

Fixes #4826
Reported-by: MrdUkk on github

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