Disable signals in libcurl.

[coryan]

TIL: libcurl uses signals to perform timeouts by default. This change
disables the signals for all the handles we create, per the
documentation on:

https://curl.haxx.se/libcurl/c/CURLOPT_NOSIGNAL.html

I will try to write a regression test later, though it might be hard
because it requires "waiting for a couple of minutes" to get the
signal.

In any case, given the documentation, and the fact that there is a bug
report (#2225) associated with this, I believe we should incorporate
these fixes.

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This change is 

[coryan]

@nickolasrossi if you have a few minutes to spare: I added an integration test to this PR that I hoped would reproduce the problem. Alas! It does not. Any suggestions on what to change would be great.

[codecov]

Codecov Report

Merging #2226 into master will increase coverage by 0.49%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2226      +/-   ##
==========================================
+ Coverage   91.99%   92.49%   +0.49%     
==========================================
  Files         307      307              
  Lines       18402    18430      +28     
==========================================
+ Hits        16929    17046     +117     
+ Misses       1473     1384      -89

Impacted Files	Coverage Δ
...le/cloud/storage/internal/curl_download_request.cc	75.18% <100%> (+0.37%)	⬆️
...ogle/cloud/storage/internal/curl_upload_request.cc	82.6% <100%> (+0.25%)	⬆️
google/cloud/storage/internal/curl_request.cc	96.77% <100%> (+0.22%)	⬆️
...le/cloud/bigtable/internal/completion_queue_impl.h	96.07% <0%> (-1.57%)	⬇️
google/cloud/bigtable/client_options.h	100% <0%> (ø)	⬆️
google/cloud/bigtable/completion_queue.h	100% <0%> (ø)	⬆️
google/cloud/examples/gcs2cbt.cc	89.6% <0%> (+72.8%)	⬆️

---

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[nickolasrossi]

Yeah, it’s tough to repro an intermittent problem... since it comes from a DNS timeout, I think you want to download from a host name that causes DNS to take a while, and set a small CURLOPT_TIMEOUT to make it time out faster.

[ghost]

Yeah, it’s tough to repro an intermittent problem... since it comes from a DNS timeout, I think you want to download from a host name that causes DNS to take a while, and set a small CURLOPT_TIMEOUT to make it time out faster.

That's a good suggestion. What do you think about something similar to this?

https://github.com/google/google-api-cpp-client/blob/d0bbe169d81a50936ec5fcea4e6dbcfb97303f13/src/googleapis/client/transport/curl_http_transport.cc#L241-L254

[coryan]

I created a clean PR with just the fixes, closing this for now. If/When I can create a regression test I will submit a separate PR.