Fix deadlock when logger tries to use correlation

[ivoanjo]

What does this PR do?

This PR fixes #3385 by preventing components initialization when calling the Datadog::Tracing.correlation API.

Instead, we use the allow_initialization: false option, which was added exactly to help break these kinds of initialization cycles.

Motivation:

This fixes a deadlock issue such as:

lib/datadog/core/configuration.rb:228:in `synchronize': deadlock; recursive locking (ThreadError)
  from lib/datadog/core/configuration.rb:228:in `safely_synchronize'
  from lib/datadog/core/configuration.rb:195:in `components'
  from lib/datadog/tracing.rb:134:in `components'
  from lib/datadog/tracing.rb:138:in `tracer'
  from lib/datadog/tracing.rb:76:in `correlation'
  from repro.rb:5:in `add'
  from logger-1.6.0/lib/logger.rb:691:in `debug'
  from lib/datadog/core/remote/component.rb:29:in `initialize'
  from lib/datadog/core/remote/component.rb:156:in `new'
  from lib/datadog/core/remote/component.rb:156:in `build'
  from lib/datadog/core/configuration/components.rb:90:in `initialize'
  from datadog-ci-0.7.0/lib/datadog/ci/configuration/components.rb:31:in `initialize'
  from lib/datadog/core/configuration.rb:248:in `new'
  from lib/datadog/core/configuration.rb:248:in `build_components'
  from lib/datadog/core/configuration.rb:89:in `block in configure'
  from lib/datadog/core/configuration.rb:230:in `block in safely_synchronize'
  from lib/datadog/core/configuration.rb:228:in `synchronize'
  from lib/datadog/core/configuration.rb:228:in `safely_synchronize'
  from lib/datadog/core/configuration.rb:84:in `configure'
  from lib/datadog/tracing/contrib/extensions.rb:64:in `configure'
  from repro.rb:9:in `<main>'

that happens when a logger is configured to also print correlation

Additional Notes:

While it may seem that I've changed the API semantics by returning an Identifier when there is no tracer, the previous code was actually misleading.

There is actually no way (that I know of) for tracer to be nil on dd-trace-rb currently. It can be disabled, but an instance will still exist.

The disabled instance, when called, returns an empty Identifier, so here I'm replicating that behavior, without needing the components to be initialized.

I guess this is open for debate? We could return nil instead in the future.

How to test the change?

This tiny snippet can be used to simulate the exact logger issue reported by the customer:

require 'ddtrace'

class TestLogger < Datadog::Core::Logger
  def add(severity, message = nil, progname = nil, &block)
    puts Datadog::Tracing.correlation
  end
end

Datadog.configure do |c|
  c.logger.instance = TestLogger.new(STDOUT)
end

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Fixes #3385