logging enhancement

[aobort]

Added:

• stacktrace to log entry
• cmd flag to enable/disable stacktrace (by default stacktrace enabled, stacktrace log level = error)

To disable stacktrace in logs, one should explicitly provide --enable-stacktrace=false flag or set corresponding env var (ENABLE_STACKTRACE=false).

Fixed:

• fallback to default logger, if context does not contain logger
• sensitive data in logged tls v1.Secret is now omitted

Fixes: #235