cmd/upspinfs: discard cached files we're done with.
cmd/upspinfs: discard cached files we're done with.

This bug kept upspinfs from discarding cached files. With this fix
we should only keep on disk open files.

Use cacheserver for better caching.

Change-Id: I128cf8578cb925af4c7b74252c1de519f3948146
Reviewed-on: https://upspin-review.googlesource.com/6901
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

upspin/upspin,upspin/upspin