extract & generalize calendar day indexes

My main goal is to support creating indexes for signals as well as
recordings. An additional goal is to just shrink db.rs a bit; it's
gotten quite large.

server/db/lib.rs

@@ -8,6 +8,7 @@ pub mod auth;
 pub mod check;
 mod coding;
 mod compare;
+pub mod days;
 pub mod db;
 pub mod dir;
 mod fs;