! newrelic: major refactor of the newrelic reporter

Most notable changes:
  - The agent connection setup is separated from the actual metrics reporting,
    this will be important in the near future when we start sending errors too.
  - The metrics subscriptions are delayed until the connection to the agent is
    established.
  - The Tick metrics buffer is only created if necessary.
  - Introduced the kamon.newrelic.max-initialize-retries and initialize-retry-delay
    settings.
  - External service calls via HTTP clients are reported as external services.
= spray: fix a incorrect trace token mismatch warning introduced by a recent refactor
! spray: correcly provide http-client segment names based on the request host.
= core,play: workaround the non thread safe calls to TrieMap.getOrElseUpdate
= core,spray,newrelic: minor formatting changes.
= core: ensure that ActorMetricsSpec uses the correct configuration.