INT-3091 Fix Concurrent (S)FTP Transfers

byte[] and String payloads were written to a temporary file.

If the same message is transferred to two different destinations
concurrently, one of the adapters could remove the temporary
file while the other was using it.

Don't use a temporary file for these payloads, simply use the
payload byte[] [or a String.getBytes()] as the InputStream
passed to the Session.write() method.

Polishing