Close the pool so that any sessions returned after the factory is
`destroy()`ed are closed.
* Call `removeAllIdleItems()` in `close()`.
* Close sessions in `SftpStreamingMessageSourceTests`.
**cherry-pick to all supported branches**
# Conflicts:
# spring-integration-core/src/test/java/org/springframework/integration/util/SimplePoolTests.java
# Conflicts:
# spring-integration-core/src/test/java/org/springframework/integration/util/SimplePoolTests.java
# spring-integration-ip/src/main/java/org/springframework/integration/ip/tcp/connection/CachingClientConnectionFactory.java
# spring-integration-sftp/src/test/java/org/springframework/integration/sftp/inbound/SftpStreamingMessageSourceTests.java
* Remove `default close()` from `Pool` interface
* Cast to `SimplePool` in the `CSF` to call the new `close()`
c95e6dcc3b