fix #187 via a dependency upgrade

I did a full `cargo upgrade` and fixed what it broke:

*   a couple things for the latest protobuf 3.0 alphas
    (note alphas don't promise API stability)
*   new minimum supported Rust version

This should have some other nice effects: parking_lot now uses inline
assembler, tokio has gotten faster, etc.

server/db/build.rs

@@ -3,13 +3,11 @@
 // SPDX-License-Identifier: GPL-v3.0-or-later WITH GPL-3.0-linking-exception.
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
-    Ok(protobuf_codegen_pure::Codegen::new()
+    Ok(protobuf_codegen::Codegen::new()
+        .pure()
         .out_dir(std::env::var("OUT_DIR")?)
         .inputs(&["proto/schema.proto"])
         .include("proto")
-        .customize(protobuf_codegen_pure::Customize {
-            gen_mod_rs: Some(true),
-            ..Default::default()
-        })
+        .customize(protobuf_codegen::Customize::default().gen_mod_rs(true))
         .run()?)
 }