Enhancements to RotatingServerAdvice

[dturanski]

Enhancement

Specific issues that depends on this are that the OOTB Sftp Source app uses this class if configured for synchronizing remote files, but uses a parallel implementation for MessageSources that simply list the contents of the remote directory. Additionally, enhancements to the source require exposing the currentDirectory in all multi-source use cases. See spring-attic/sftp#60

1. RotatingServerAdvice delegates to StandardRotationPolicy which assumes that the MessageSource is one of two types, and downcasts the MessageSource to the appropriate type to perform some specific actions. This should be decoupled from the generic server rotation logic to allow use with other type of message sources.
2. Expose the currentDirectory()of RotationPolicy so subclasses of RotatingServerAdvice can access it.