Connect tracer with external application's logging facilities

[cgilmour]

This PR adds logging functionality that allows the tracer library to emit log messages using the application's logger.
When initializing the tracer, the application provides a log function that is used as a bridge between the library and application. Then if the tracer needs to write a log message, it calls the application-provided log function with a log level and message.

This also improves the startup logging, so instead of writing to a file, it is written to the application's logs.

There's two sets of methods to use when logging something.
Log should be used for actionable information.
Trace should be used for detailing the call flow and internal decision-making.

The logger is instantiated as either a StandardLogger or VerboseLogger. The StandardLogger provides an empty implementation for the Trace methods, which means there's no significant overhead to calling them - it becomes almost a no-op.
The DD_TRACE_DEBUG environment variable needs to be set to true to use the VerboseLogger and have the performance impact of the detailed logging.

There are still occasions where stderr is used, and places where Trace could be used, and this will be covered in future PRs.