#25 OutOfMemoryError; clean up closed sessions

[Chris Mair]

Get java.lang.OutOfMemoryError: Java heap space - after running for a long time (and opening lots of sessions).

See:
https://sourceforge.net/p/mockftpserver/discussion/748298/thread/55aebe06/
https://sourceforge.net/p/mockftpserver/bugs/24/

[Chris Mair]

Session: Added boolean Session.isClosed().
AbstractFtpServer: Clean up closed sessions.

Fixed for MockFtpServer 2.6