Custom timestamps for output logging

[jwbonner]

It would be useful to be able to log outputs using a timestamp override. Use cases include logging vision data with an offset timestamp (accounting for latency) or logging high-frequency odometry.

Needs to be restricted to outputs only (unless we add a built-in queuing system for inputs).

[jwbonner]

With this change, it would also be helpful to make the logging methods thread-safe. (#174)