Allow absolute paths in SFTP client requests

The SFTP client always trims the leading / in the request paths. Consequently, it is impossible to make an SFTP request with an absolute path. While making requests with an absolute path is generally a bad practice (as it may allow a remote agent to access parts of the filesystem that they shouldn't access), we sometimes don't have a choice.

Thus, the SFTP client should be changed to allow both relative and absolute remote filepaths (this may require changing the way filepaths are stored in the database).

The R66 & HTTP client may need to be changed as well if these restrictions also apply to them.