Keep clone files and oversized responses out of the cache

The dumb transport reads files that already sit on the disk, so a pack
copied into a slot cost that disk twice and the request a second write
of every byte. A snapshot took a slot whatever its size, so a visitor
naming distinct refs and ids could fill the cache root with archives.
`cache-max-slot-size`, 64 MB unless set, now serves a larger response
from the lock file and drops it, along with any expired copy it would
have replaced.

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