INT-3266 (S)FTP Close Dirty Cached Sessions

If an exception occurs on a session it should be physically closed
and not resused because its state is indeterminate and the next
operation might fail.

Note: The booleans within CachedSession do not need to be volatile
because it is a short-lived object only used by the current thread.

Also fixes the assertion message in RFT.get().

JIRA: https://jira.springsource.org/browse/INT-3266