Block-Flow: integrate actual Gate-implementation

Each Epoch in the memory manager holds a Gate in the first slot;
after the logic for Gate-activation is worked out now, we can switch
to using this actual logic to determine when an Epoch can be released

src/vault/gear/activity.hpp

@@ -241,6 +241,10 @@ namespace gear {
         {
           int     rest;     ///< alive while rest > 0
           Instant dead;     ///< alive while time < dead
+          
+          bool isDead (Time now)  const { return dead <= now;}
+          bool isHold ()          const { return rest > 0;   }
+          bool isFree (Time now)  const { return not (isHold() or isDead(now)); }
         };
       
       /** Time window to define for activation */

src/vault/gear/block-flow.hpp

@@ -260,8 +260,8 @@ namespace gear {
             bool
             isAlive (Time deadline)
               {
-                /////////////////////////////////////////////OOO preliminary implementation ... should use the GATE-Activity itself
-                return this->deadline() > deadline;
+                return data_.condition.isHold() // NOTE: expected callback keeps alive
+                    or not data_.condition.isDead (deadline);
               }
             
             size_t