Release v3.7.0

Changes

• When Config.Transport is nil, no longer use the http.DefaultTransport
  when communicating with the New Relic backend. This addresses an issue with
  shared transports as described in golang/go#33006.

• If a timeout occurs when attempting to send data to the New Relic backend,
  instead of dropping the data, we save it and attempt to send it with the
  next harvest. Note data retention limits still apply and the agent will
  still start to drop data when these limits are reached. We attempt to keep
  the highest priority events and traces.