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.

src/stream.rs

@@ -34,8 +34,7 @@ use failure::{Error, bail};
 use ffmpeg;
 use lazy_static::lazy_static;
 use log::{debug, info, warn};
-use std::os::raw::c_char;
-use std::ffi::{CStr, CString};
+use std::ffi::CString;
 use std::result::Result;
 use std::sync;