Allow toggling background sending on the fly

[st0012]

In some cases, being able to disable background sending globally is crucial for both users and the SDK itself.

For example, Rails supports having different modes for running the application (server, console, runner, task...etc). In the runner or task mode, the process is killed right after script is returned, usually before the background worker have the chance to send the events. Here's an example issue: #1324

Although Rails provides callbacks for different modes, they're ran after the SDK initialization. And under the current setup, it's not possible to temporarily bypass background sending globally past the SDK initialization stage. So we can't disable the background workers in those callbacks.

To solve the problem, this commit makes it possible to bypass background worker even after it's been initialized.

[codecov-commenter]

Codecov Report

Merging #1447 (6086d17) into master (5ce32c1) will increase coverage by 0.61%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1447      +/-   ##
==========================================
+ Coverage   98.22%   98.83%   +0.61%     
==========================================
  Files         213      118      -95     
  Lines       10034     6202    -3832     
==========================================
- Hits         9856     6130    -3726     
+ Misses        178       72     -106     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/client.rb	97.18% <100.00%> (ø)
...ntry-ruby/spec/sentry/client/event_sending_spec.rb	99.51% <100.00%> (+0.01%)	⬆️
sentry-raven/lib/raven/base.rb
...en/lib/raven/processor/removecircularreferences.rb
sentry-raven/lib/raven/interface.rb
sentry-raven/lib/raven/utils/request_id.rb
...entry-raven/lib/raven/integrations/rack-timeout.rb
...b/raven/integrations/sidekiq/cleanup_middleware.rb
sentry-raven/spec/raven/integrations/rack_spec.rb
sentry-raven/lib/raven/event.rb
... and 87 more

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