Add a file connector sink.

[leukhe]

First it should be a reverse of the file source connector:
-one file per message
-option to put the date from metadata of message on date of file.
-atomic write, write to tempdir on same mount, and move file to destination
-if write fails, don't consume message

And later features:
Write to single file, roll to next file after x size/lines/time
Call an executable to process a file.