Fix reading of files where fields may contain many newlines

[st-pasha]

I tested that this fix allows fread to correctly parse the jigsaw-toxic-comments and avito-demand-prediction kaggle datasets.

The bug is resolved by removing the safeguard that would stop reading a field after encountering 100 newlines inside it. This safeguards breaks those use cases where the used does have a dataset with fields containing many newlines (eg. emails, extended descriptions, user comments on the web, etc.)

At first I thought of merely raising the limit higher -- say, to 10000. But that would merely make fread fail less often, but wouldn't eliminate the problem altogether. I also thought of making it an fread option exposed to the user. That would have added more complexity (such as throwing an exception suggesting the user to increase that newline limit), and then the first thing the user would probably do is to increase that limit anyways. So in the end that increased complexity would have served no purpose whatsoever...

The reason why the limit was there in the first place was so that if the user didn't quote their fields correctly, and there was just a single quote in the whole huge file, we didn't want to spend time reading the entire file before trying a more liberal QR. But I feel reading a well-formed CSV file correctly is much more important than the possibility of wasting some time when reading an ill-formed CSV... Because in reality there is no limit on how many newlines you may have in a text field. It could be a billion (although once a single field reaches in size 2^31 bytes, everything will break anyways :).

Closes #2395
Closes #2600

[codecov-io]

Codecov Report

Merging #2800 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2800      +/-   ##
==========================================
- Coverage   93.49%   93.48%   -0.01%     
==========================================
  Files          61       61              
  Lines       12367    12364       -3     
==========================================
- Hits        11562    11558       -4     
- Misses        805      806       +1

Impacted Files	Coverage Δ
src/fread.c	97.95% <ø> (-0.09%)	⬇️

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[mattdowle]

Yep, completely agree. Great.