INT-3047 SFTP Multiplex Over Single Connection

Enable the use of multiple JSch channels over a
single connection.

JIRA: https://jira.springsource.org/browse/INT-3047

INT-3047: Polishing locks logic

INT-3047 SFTP Reset Cache; Quiesce Shared Sessions

Support resetCache() on the CachingSessionFactory
- reset a shared JSch session so it is reestablished on next use
- immediately close idle sessions
- close in-use sessions as they are returned
- close the channel when an SftpSession is closed and a shared JSch is being used
- physically close a shared JSch session only when the last channel is closed

Polishing

Forgot to save the ftp.xml.

Change CachedSession.epoch to created.

INT-3047 Polishing - PR Comments

- Also add missing docs for `preserve-timestamp`

INT-3047 Polish - PR Comments

Also change epoch to nanoseconds and use a simple != comparison with the
epoch in which a session was created.