reorganize database crate.

split database Cork into unit.

split database migrations from mod.rs

Signed-off-by: Jason Volk <jason@zemos.net>

src/database/key_value/globals.rs

@@ -10,7 +10,7 @@ use ruma::{
 };
 
 use crate::{
-	database::{abstraction::Cork, KeyValueDatabase},
+	database::{Cork, KeyValueDatabase},
 	service, services, utils, Error, Result,
 };