Fixes: https://github.com/spring-projects/spring-integration/issues/3249
When the `CachingSessionFactory` is configured with small enough pool
and it is very likely that dead lock may happen when `RemoteFileTemplate.send()`
is used.
The problem happens when we reach the `RemoteFileTemplate.exists()` call
which is done from the internal method called from already pulled from cache
`Session`
* Fix `RemoteFileTemplate` to use a `session.exists()` instead on the provided
into the method `Session`
* Demonstrate the problem in the `SftpRemoteFileTemplateTests.testNoDeadLockOnSend()`
**Cherry-pick to 5.2.x, 5.1.x & 4.3.x**
Fix RemoteFileOutboundGWTests for the proper mock
c00c319acb