dcache-finegrained-locks-20011121

dcache-finegrained-locks-20011121

This patch implements more fine-grained locking for dcache entries.
The main advantage is that multiple chunks of the same file can be
fetched at once.  This means that an incorrectly-guessed prefetch
won't block other fetches, prefetches of multiple chunks can occur
in parallel, and multiple processes sharing the same file can read
from different parts of the file at once.