fuse/impl_a: fix a suspected bug in caching

fuse/impl_a: fix a suspected bug in caching

from my examination of the tahoe_fuse ('impl_a') code, it looks like
the intention is to cache the file contents in memory while it's open,
since it does in fact do that.  however it looks like it also ignored
that cache entirely, and made an individual tahoe webapi GET request
for each and every read() operation regardless of the relative size of
the read block and the file in question.

this changes that to make read() use the data in memory rather than
fetch the data over again.   if there's something more subtle going
on, please let me know.