Feature request: allow hasher passthrough to be disabled if not included in supported hash types

Hello,

Please forgive the title; I couldn't think of a good way to word it.

When you use the hasher backend, and you request a hash type that is not saved in the database, the request is passed through to the underlying filesystem. This can occasionally cause some problems.

For example, if you have Hasher set to only save SHA1 hashes, and you then use the dedupe command, the check will take a long time because Rclone is going through and getting MD5 hashes for each file.

If we could disable the passthrough, I think dedupe would resort to using the hash type that Hasher is using.