Increase FUSE read_size to 1024.

From https://github.com/borgbackup/borg/pull/480 discussion:

Did you try 1024 (linux cache block size) or 4096 (internal sector size of bigger
hdds, also used in msgpack fallback.py as lower bound, see link)?

I've tested different values - 512 and 1024 are slightly better than 4096 in my case.

read_size = 1    ls -laR: 75.57 sec
read_size = 64   ls -laR: 27.81 sec
read_size = 512          ls -laR: 27.40 sec
read_size = 1024         ls -laR: 27.20 sec
read_size = 4096         ls -laR: 30.15 sec
read_size = 0    ls -laR: 442.96 sec (default)

OK, maybe we should go for 1024 then. That happens to be < MTU size, so in case someone works on NFS
(or other network FS) we will have less reads, less network packets, less latency.

borg/fuse.py

@@ -28,7 +28,7 @@ class ItemCache:
 
     def get(self, inode):
         self.fd.seek(inode - self.offset, io.SEEK_SET)
-        return next(msgpack.Unpacker(self.fd, read_size=512))
+        return next(msgpack.Unpacker(self.fd, read_size=1024))
 
 
 class FuseOperations(llfuse.Operations):