message: service.S3: clean up S3
diff: @@ -64,10 +64,12 @@ namespace elle
  /// Use of this should be optimised based on S3 pricing:
  /// http://aws.amazon.com/s3/pricing/
  S3(Credentials const& credentials);
  +
  + using QueryCredentials = std::function<auto (bool) -> Credentials>;
  /// Version taking a function able to refresh credentials.
  /// Bool argument is true on the first call, false on call caused by
  /// expiration of current cached creds.
  - S3(std::function<auto (bool) -> Credentials> query_credentials);
  + S3(QueryCredentials query_credentials);
  /*-----------.
  | Operations |
  @@ -161,7 +163,7 @@ namespace elle
  hostname(Credentials const& credentials,
  boost::optional<std::string> override_host = {}) const;
  ELLE_ATTRIBUTE(Credentials, credentials);
  - ELLE_ATTRIBUTE(std::function<Credentials(bool)>, query_credentials);
  + ELLE_ATTRIBUTE(QueryCredentials, query_credentials);
  /*--------.
  | Helpers |
  