[SFTP Adapters]Allow clearing unprocessed files in AbstractRemoteFileStreamingMessageSource

[kkosiacki]

Expected Behavior

When I'm using RotatingServerAdvice when pooling files from different remote, I want to have ability to remove unprocessed files from queue, to avoid trying to get file from remote when Session is rotated.

Current Behavior

When you have DelegatingSessionFactory with RotatingServerAdvice and there are unprocessed files in queue (see toBeReceived in AbstractRemoteFileStreamingMessageSource), when you call doReceive MessageSource will try to retrieve file from queue and it will be success or not depends on Session return from DelegatingSessionFactory. If rotation happened and 'new' session is not able to retrieve file from remote, exception is thrown (access denied) that clears that internal queue.

Context
I want to add public method on AbstractRemoteFileStreamingMessageSource thats allow developer to decide if he wants to clean this queue (before doReceive). It means that you can write own RotatingPolicy that will clear toBeReceived queue if necessary.