Introduce a dedicated log handler

[ChaseWiseman]

Let's aim to break out all logging functionality to a dedicated handler. Potential improvements:

• Support WooCommerce 3.0's set of standardized log levels
• [Gateways] Update the debug log setting to support the different levels
• [Gateways] Determine when to log according to the setting, as it relates to transaction events
• Support the WC 3.0 DB logger

Questions

• Should we add email triggering for certain log levels?

[maxrice]

add email triggering for certain log levels

I like this, would need to decide on specifics of who we send a notification to and whether we can rate-limit sending.