Add correlation identifier

[delner]

In many cases, such as logging, it may be useful to correlate trace IDs to other events or data streams, for easier cross referencing. This pull request adds active_correlation_ids to the tracer which produces a Datadog::Correlation::Identifier for the currently active trace, which can then be used to decorate other data sources.

An example of this for the purpose of logging:

require 'ddtrace'
require 'logger'

logger = Logger.new(STDOUT)
logger.progname = 'my_app'
logger.formatter  = proc do |severity, datetime, progname, msg|
  # Returns Datadog::Correlation::Identifier
  ids = Datadog.tracer.active_correlation_ids
  "[#{datetime}][#{progname}][#{severity}][dd.trace_id=#{ids.trace_id} dd.span_id=#{ids.span_id}] #{msg}\n"
end

# When a trace is active...
Datadog.tracer.trace('logging.example') do
  # And a warning is produced...
  logger.warn('This is a warning.')

  # Prints:
  # [2018-12-18 22:42:25 +0000][my_app][WARN][dd.trace_id=5963550561812073440 dd.span_id=2232727802607726424] This is a warning.
end

# When no trace is active...
# And a warning is produced...
logger.warn('This is a warning.')

# Prints:
# [2018-12-18 22:44:19 +0000][my_app][WARN][dd.trace_id= dd.span_id=] This is a warning.

[brettlangdon]

Looks great!