Combine the two Lock classes into one class (#1126)

* Combine the two Lock classes into one class

This allows us to remove the DAO and to just treat locks the same as we
would treat any other object -- generically grabbing them from the
transaction manager.

We do not need to be concerned about the changeover between Datastore
and SQL because we assume that any such changeover will require
sufficient downtime that any currently-valid acquired locks will expire
during the downtime. Otherwise, we could get into a situation where an
action has acquired a particular lock in Datastore but not SQL.

db/src/main/resources/sql/schema/db-schema.sql.generated

@@ -501,11 +501,11 @@
 
     create table "Lock" (
        resource_name text not null,
-        tld text not null,
+        scope text not null,
         acquired_time timestamptz not null,
         expiration_time timestamptz not null,
         request_log_id text not null,
-        primary key (resource_name, tld)
+        primary key (resource_name, scope)
     );
 
     create table "PollMessage" (