Provide way to see unified log of message events

[twschiller]

Context

• Browser messaging is a distributed system with some unintuitive message routing behavior
• During development/debugging, it would be nice to see the exact sequence of message events, possibly using a tool for distributed systems like https://bestchai.bitbucket.io/shiviz/
  -Depending on how it's implemented, for testing, we could even write assertions like "happened before", etc.

Implementation Sketch

• Create a "trace" flag to control whether event tracing is enabled
• Send event metadata to a common location, e.g., a text file, IDB database, or to the background page
  • For each message received, log timestamp, who it was received from, and type
  • For each message sent, log timestamp, the target it was sent to, etc.
  • For each message responded to, log timestamp, who it was received from, success/error, etc.