sftp upload: expire to advance state machine

When using the multi_socket API to do SFTP upload, it is important that
we set a quick expire when leaving the SSH_SFTP_UPLOAD_INIT state as
there's nothing happening on the socket so there's no read or write to
wait for, but the next libssh2 API function needs to be called to get
the ball rolling.

This is one of the problems detected by test 582.

Reported by: Henry Ludemann <misc@hl.id.au>