Use fixed-size directory meta files

Add a new schema version 5; now 4 means the directory meta may or may
not be upgraded.

Fixes #65: now it's possible to open the directory even if it lies on a
completely full disk.

db/check.rs

@@ -183,7 +183,7 @@ fn read_dir(path: &str, opts: &Options) -> Result<Dir, Error> {
         let f = e.file_name();
         let f = f.as_bytes();
         match f {
-            b"meta" | b"meta-tmp" => continue,
+            b"meta" => continue,
             _ => {},
         };
         let id = match dir::parse_id(f) {