Before this change if you specified --hash MD5 in rclone lsf it would
calculate all the hashes and just return the MD5 hash which was very
slow on the local backend.
Likewise specifying --hash on rclone lsjson was equally slow.
This change introduces the --hash-type flag (and corresponding
internal parameter) so that the hashes required can be selected in
lsjson.
This is used internally in lsf when the --hash parameter is selected
to speed up the hashing by only hashing with the one hash specified.
Fixes#4181
f37af9afec