XD-1677 Add 'level' and 'expression' to Log Sink

JIRA: https://jira.spring.io/browse/XD-1677

Use `--expression=#root` to log entire message.

Polishing

Offline comments...

 - change default level to INFO
 - add 'xd.sink' as the log category prefix
 - set the xd.sink log category level to INFO