This code change is one step in the higher-level Logging feature, see: ros2/ros2#425 for others

This adds a Logger class, the node->get_logger() method, and the rclcpp::get_logger(name) free function for non-node loggers.

Logging macros must now have a logger object passed as the first argument (instead of a name as previously accepted). I added

to give something more useful than a wall of macro errors if users try to pass a string: open to other implementation suggestions.

The Logger class is based off a rough implementation initially by @wjwwood. The node's logger is implemented as an interface on @Karsten1987's suggestion to support the use case of composing a lightweight Node that does not include or "pay for" the logger interface. Additionally, even if using the logger interface, logging functionality can be minimised with the preprocessor directive RCLCPP_LOGGING_ENABLED set to false.

Todo before merge:

• remove logger namespace following Escalate more namespaces e.g. rclcpp::Service #410 (comment)
• do whatever needs to be done for lifecycle_nodes to have a logger (haven't investigated yet)

Other todos listed in the code are planned as follow-up PRs.

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