GH-3370: Remove synchronized from RemoteFileUtils (#3380)

* 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`

spring-integration-sftp/src/test/java/org/springframework/integration/sftp/outbound/SftpOutboundTests.java

@@ -219,7 +219,7 @@ public class SftpOutboundTests {
 		final List<String> madeDirs = new ArrayList<>();
 		doAnswer(invocation -> {
 			madeDirs.add(invocation.getArgument(0));
-			return null;
+			return true;
 		}).when(session).mkdir(anyString());
 		handler.handleMessage(new GenericMessage<>("qux"));
 		assertThat(madeDirs.size()).isEqualTo(3);