Revert "core: do not spawn jobs or touch other units during coldplugging"

This reverts commit 6e392c9c45.

We really shouldn't invent external state keeping hashmaps, if we can
keep this state in the units themselves.

src/core/unit.h

@@ -303,14 +303,8 @@ struct UnitVTable {
         int (*load)(Unit *u);
 
         /* If a lot of units got created via enumerate(), this is
-         * where to actually set the state and call unit_notify().
-         *
-         * This must not reference other units (maybe implicitly through spawning
-         * jobs), because it is possible that they are not yet coldplugged.
-         * Such actions must be deferred until the end of coldplug bу adding
-         * a "Unit* -> int(*)(Unit*)" entry into the hashmap.
-         */
-        int (*coldplug)(Unit *u, Hashmap *deferred_work);
+         * where to actually set the state and call unit_notify(). */
+        int (*coldplug)(Unit *u);
 
         void (*dump)(Unit *u, FILE *f, const char *prefix);
 
@@ -546,7 +540,7 @@ int unit_deserialize(Unit *u, FILE *f, FDSet *fds);
 
 int unit_add_node_link(Unit *u, const char *what, bool wants);
 
-int unit_coldplug(Unit *u, Hashmap *deferred_work);
+int unit_coldplug(Unit *u);
 
 void unit_status_printf(Unit *u, const char *status, const char *unit_status_msg_format) _printf_(3, 0);