Support category-based rate limiting

[st0012]

Sentry rate limits different types of events. And when rate limiting is enabled, it sends back a 429 response to the SDK. Currently, the SDK would then raise an error like this:

Unable to record event with remote Sentry server (Sentry::Error - the server responded with status 429
body: {"detail":"event rejected due to rate limit"}):

This change improves the SDK's handling on such responses by:

• Not treating them as errors, so you don't see the noise anymore.
• Halting event sending for a while according to the duration provided in the response. And warns you with messages like:

Envelope [event] not sent: Excluded by random sample

Closes #1072

[codecov-io]

Codecov Report

Merging #1336 (1126dcd) into master (86dcf3c) will increase coverage by 0.63%.
The diff coverage is 99.50%.

@@            Coverage Diff             @@
##           master    #1336      +/-   ##
==========================================
+ Coverage   98.08%   98.71%   +0.63%     
==========================================
  Files         208      114      -94     
  Lines        9173     5538    -3635     
==========================================
- Hits         8997     5467    -3530     
+ Misses        176       71     -105     

Impacted Files	Coverage Δ
sentry-ruby/spec/sentry/hub_spec.rb	100.00% <ø> (ø)
...try/transport/http_transport_rate_limiting_spec.rb	99.17% <99.17%> (ø)
sentry-ruby/lib/sentry/transport.rb	98.41% <100.00%> (+1.19%)	⬆️
sentry-ruby/lib/sentry/transport/http_transport.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/configuration_spec.rb	98.61% <100.00%> (ø)
sentry-ruby/spec/sentry/transport_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry_spec.rb	100.00% <100.00%> (ø)
sentry-raven/spec/raven/processors/cookies_spec.rb
sentry-raven/lib/raven/context.rb
sentry-raven/lib/raven/processor/cookies.rb
... and 93 more

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[jan-auer]

Thank you! Please see the comments below. There is one request for change in the code checking for cached limits, and a few suggestions and clarifying questions.