SFTP sizeCheckWaitTime should be applied per poll cycle instead of per file

SFTP transport defines sizeCheckWaitTime attribute which is used to verified that a file was not updated in the last X milliseconds, so its OK to process it.

On every polling cycle, the SFTP gets the file names form the server and then, for every file, it will wait for sizeCheckWaitTime ms in order to check that the file was not changed.
The way this attribute is used causes a big downtime when there are many files to be processed and/or the value of sizeCheckWaitTime is not negligible.

The correct way to use this attribute would be to list every file, get the intial file size, wait for the sizeCheckWaitTime interval and the get the file size again and check which files have changed or not.