DefaultSftpSessionFactory.getSession() race condition when isSharedSession

When the isSharedSession is used for the DefaultSftpSessionFactory, there is some race condition window when we can call the target this.jschSession.connect() several times and end up with the session is already connected.

Wrap sftpSession.connect() to the this.sharedSessionLock.readLock().lock() when isSharedSession to protect from that race condition.