* logging: added logger wrappers for Broadcast and Prefix
* nit: moved max hash size to a named constant
* content: added internal logger
* content: replaced context-based logging with explicit Loggers
This will capture the logger.Logger associated with the context when
the repository is opened and will reuse it for all logs instead of
creating new logger for each log message.
The new logger will also write logs to the internal logger in addition
to writing to a log file/console.
* cli: allow decrypting all blobs whose names start with _
* maintenance: added logs cleanup
* cli: commands to view logs
* cli: log selected command on each write session
