[SecurityFix] Change DefaultStreamLoader start log from info to debug…

[fabrizziocht]

… to prevent exposing starrocks credentials

What type of PR is this：

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Which issues of this PR fixes ：

Fixes #415

Problem Summary(Required) ：

This change will prevent to expose starrocks credentials in any logger system, for example if you send the logs to any server like datadog, newrelic or a simple stdout, it is easy to collect the database connection, username and password.
The log is divided in two parts:

• A simple log info message that notifies that the loadstream started.
• In case the log level is debug then you can print all properties in detail (assumming that in production scenarios debug should not be used).

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr will affect users' behaviors
• This pr needs user documentation (for new or modified features or behaviors)
• I have added documentation for my new feature or new function

[CLAassistant]

All committers have signed the CLA.