chore: improve logs

[juleswritescode]

1. Adds a configurable bunyan log format (default is still hierarchical), configurable via PGT_LOG_KIND env var
2. Adds timestamps to hierarchical log format
3. Makes log level configurable via PGT_LOG_LEVEL env var, default stays the same
4. Cleans up log journey by removing logged fields that we do not use at runtime

[juleswritescode]

@psteinroe The default log level is now info – if we ask issue reporters to copy their log files, maybe the default should be debug. wdyt?

[psteinroe]

thanks for the flexibility!