Upgrade to leveled logging

[AlexJuca]

Context

Production services need structured, leveled logging with timestamps and enough context to debug incidents. FKVS currently logs mostly through printf/fprintf and compile/runtime verbosity flags.

Scope

• Add log levels: DEBUG, INFO, WARN, ERROR.
• Include timestamp, level, subsystem, and useful context such as client fd/address where safe.
• Make log level configurable.
• Avoid logging raw values by default to prevent sensitive data exposure.
• Support stdout/stderr logging for containers and file/syslog mode if needed for daemon deployments.
• Replace ad hoc logging in server startup, event loops, command errors, config parsing, and shutdown.

Acceptance criteria

• Logs are consistent and parseable enough for production operations.
• Verbose protocol dumps are opt-in and safe by default.
• Errors include actionable context without leaking key/value contents unnecessarily.
• Logging behavior is documented.