SFTP: too many LSTAT calls

While looking at an alternate solution for SSHD-1217 I noticed that the AbstractSftpSubsystemHelper makes several LSTAT calls for a single FileSystem.readAttributes() invocation.

Basically it makes one LSTAT call per supported attributes view; only to collect the returned items then in one single map anyway.

This doesn't make any sense. SFTP file attributes are a single, defined structure. If AbstractSftpSubsystemHelper.getAttributes() needs to collect all these different views (BasicFileAttributes, PosixFileAttributes, and so on) on these underlying SFTP attributes, then all these views should build upon the same single SftpClient.Attributes gotten exactly once.

This could be solved with careful temporary caching of SFTP attributes on the SftpPath once read while in AbstractSftpSubsystemHelper.getAttributes() and clearing that cache at the end.

The problem is more general, though. The code in several places makes up-front checks whether a file exists, is a directory, and so on. This is a questionable pattern anyway, since the result of a Files.exists() is outdated immediately; one cannot rely on the file being still there on the next access. With an SftpPath, this exists() call is an (L)STAT remote call getting the attributes. Now look at AbstractSftpSubsystemHelper.resolveFileAttributes: here getting the attributes themselves is so guarded, so it makes at least two LSTAT calls.

This should also be improved.