logging: Use global logging config everywhere

[mitjat]

This PR has two related small improvements to logging:

• We used NewDefaultLogger in a few places before; this is not great as the default logger does not reflect the .yml config (e.g. the desired logging level, the format, etc). We now use RootLogger (which is derived from the config) instead.
• pogreb was logging straight to stderr, so all its info-level logs were interpreted as errors by GCP, and stood out in the log browser. We now route pogreb logs through our JSON-formatted logger, at INFO level.