Refresh adapter's logger model to match the pkg/log pattern. (#4742)
Refresh adapter's logger model to match the pkg/log pattern. (#4742)

- The VerbosityLevel function is gone, replaced by InfoEnabled, DebugEnabled,
ErrorEnabled, and WarnEnabled.

- Added the Debugf function for debug-level logging.
istio/pkg,istio/pkg,istio/pkg,istio/pkg,istio/pkg