* GH-3370: Remove synchronized from RemoteFileUtils
Fixes https://github.com/spring-projects/spring-integration/issues/3370
The `synchronized` on the `RemoteFileUtils.makeDirectories()` makes an application too
slow, especially when we deal with different paths in different sessions
* Remove the `synchronized` from that method and rework `SftpSession.mkdir()`
to return `false` when "A file cannot be created if it already exists" exception
is thrown from the server.
Essentially make an `exists()` call to be sure that an exception is really related
to "file-already-exists" answer from the server
**Cherry-pick to 5.3.x, 5.2.x & 4.3.x**
* * Re-throw an exception in the `SftpSession.mkdir()`
when error code is not `4` or remote dir does not exist
* * Check `session.mkdir()` result in the
`RemoteFileUtils` to throw an `IOException` when `false`
* * Fix mock test to return `true` for `mkdir` instead of `null`
# Conflicts:
# spring-integration-file/src/main/java/org/springframework/integration/file/remote/RemoteFileUtils.java
# spring-integration-file/src/main/java/org/springframework/integration/file/remote/gateway/AbstractRemoteFileOutboundGateway.java
# spring-integration-file/src/test/java/org/springframework/integration/file/remote/gateway/RemoteFileOutboundGatewayTests.java
# spring-integration-sftp/src/test/java/org/springframework/integration/sftp/outbound/SftpOutboundTests.java
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