Fix the lock contention stall during cover art load, by adding a separate modify...

Fix the lock contention stall during cover art load, by adding a separate modify mutex for the buffer. Operations that modify contents of a buffer entry can still proceed, but ones that add or remove buffer entries, or move them
in memory, will still block. Some members of struct memory_handle also need an earlier init in bufopen to make sure that buffer stats aren't trashed.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@20932 a1c6a512-1295-4272-9138-f99709370657