upgrade to 2018 Rust edition

This is mostly just "cargo fix --edition" + Cargo.toml changes.
There's one fix for upgrading to NLL in db/writer.rs:
Writer::previously_opened wouldn't build with NLL because of a
double-borrow the previous borrow checker somehow didn't catch.
Restructure to avoid it.

I'll put elective NLL changes in a following commit.

db/dir.rs

@@ -32,11 +32,11 @@
 //!
 //! This includes opening files for serving, rotating away old files, and saving new files.
 
-use db::CompositeId;
+use crate::db::CompositeId;
 use failure::{Error, Fail};
 use libc::{self, c_char};
 use protobuf::{self, Message};
-use schema;
+use crate::schema;
 use std::ffi;
 use std::fs;
 use std::io::{self, Read, Write};