Replace chrono dependency by time

[AngelicosPhosphoros]

chrono has not been released for very long time (1 year already).
Also, it has security error: https://rustsec.org/advisories/RUSTSEC-2020-0159.html

Also, this would reduce dependencies because chrono uses very old time version so many binaries which use gotham ends with 2 versions of time linked.

[AngelicosPhosphoros]

There is a new contributor to chrono so they could fix chrono in future.
chronotope/chrono#606 (comment)

[AngelicosPhosphoros]

Also, this change reduce complexity of Timing type because there is no more Invalid state.

[AngelicosPhosphoros]

Also, I suggest to add std::time::Instant field to Timer struct to get more valid durations.
Old and current solutions can lead to negative durations which is not very good. This can happen, for example, after NTP sync.

So we would use time to log start time but we would use Instant to get duration of requests.

[codecov]

Codecov Report

Merging #578 (cc6dfdc) into master (097822e) will decrease coverage by 0.05%.
The diff coverage is 55.55%.

❗ Current head cc6dfdc differs from pull request most recent head 3d5bb6d. Consider uploading reports for the commit 3d5bb6d to get more accurate results

@@            Coverage Diff             @@
##           master     #578      +/-   ##
==========================================
- Coverage   85.84%   85.78%   -0.06%     
==========================================
  Files         112      112              
  Lines        5770     5781      +11     
==========================================
+ Hits         4953     4959       +6     
- Misses        817      822       +5     

Impacted Files	Coverage Δ
gotham/src/middleware/logger.rs	0.00% <0.00%> (ø)
gotham/src/helpers/timing.rs	66.66% <71.42%> (+66.66%)	⬆️
examples/hello_world_until/src/main.rs	84.90% <0.00%> (-11.33%)	⬇️
gotham/src/middleware/session/backend/memory.rs	86.73% <0.00%> (-4.09%)	⬇️

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[msrd0]

Also, I suggest to add std::time::Instant field to Timer struct to get more valid durations.

I was just thinking exactly that. That also removes the Invalid case as it guarantees monotonicity.

[msrd0]

Thanks