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.

spring-integration-core/src/main/java/org/springframework/integration/util/SimplePool.java

@@ -192,7 +192,7 @@ public class SimplePool<T> implements Pool<T> {
 		}
 		else if (this.callback.isStale(item)) {
 			if (logger.isDebugEnabled()) {
-				logger.debug("Received a stale item, will attempt to get a new one.");
+				logger.debug("Received a stale item " + item + ", will attempt to get a new one.");
 			}
 			doRemoveItem(item);
 			item = doGetItem();
@@ -241,6 +241,9 @@ public class SimplePool<T> implements Pool<T> {
 	}
 
 	private void doRemoveItem(T item) {
+		if (logger.isDebugEnabled()){
+			logger.debug("Removing " + item + " from the pool");
+		}
 		this.allocated.remove(item);
 		this.inUse.remove(item);
 		this.callback.removedFromPool(item);