Improvements to API and logging/debugging

[asgrim]

• Hardens Agent API to require Logger in ::fromConfig()
• Removes deprecated API (Agent::fromDefaults())
• Makes Agent::__construct into a private method (forces using ::fromConfig())
• Return the data that was read from the socket in Connector (for logging, primarily)
• Adds a bunch of extra logging everywhere
• Adds docs on how to turn off all the logging

[asgrim]

Note this PR breaks BC in several ways (noted in the changelog) which means, if merged, the next version will be 2.0.0, assuming we are following semver.