Merged PR 278: Enable the red agent to vary its start node

## Summary
- Made the data manipulation red agent be able to choose between the two clients to start operating on
- changed the attacker name in the config to 'data_manipulation_attacker' to because it is no longer tied to any client
- Updated the documentation notebook accordingly.
- Fixed a bug where the database client made a new connection every time it sent a SQL query (it tries to reuse its most recent one instead)
- Fixed a bug where link loads were not being cleared between episodes (?)

**warning** - the green agents are not working properly after reset right now, but I'm gonna fix this in the next ticket where I refactor episode reset.

## Test process
- unit tests pass
- UC2 notebook passes with both clients. (currently this doesn't work after an episode reset, but the very next thing I'm gonna work on is refactoring the reset, so I don't want to waste time fixing this.)

## Checklist
- [x] PR is linked to a **work item**
- [x] **acceptance criteria** of linked ticket are met
- [x] performed **self-review** of the code
- [x] written **tests** for any new functionality added with this PR
- [ ] updated the **documentation** if this PR changes or adds functionality
- [ ] written/updated **design docs** if this PR implements new functionality
- [x] updated the **change log**
- [x] ran **pre-commit** checks for code style
- [n] attended to any **TO-DOs** left in the code

Related work items: #2232

src/primaite/simulator/network/hardware/base.py

@@ -548,6 +548,11 @@ class Link(SimComponent):
     def __str__(self) -> str:
         return f"{self.endpoint_a}<-->{self.endpoint_b}"
 
+    def apply_timestep(self, timestep: int) -> None:
+        """Apply a timestep to the simulation."""
+        super().apply_timestep(timestep)
+        self.current_load = 0.0
+
 
 class Node(SimComponent):
     """

src/primaite/simulator/system/applications/database_client.py

@@ -196,7 +196,11 @@ class DatabaseClient(Application):
             return False
 
         if connection_id is None:
-            connection_id = str(uuid4())
+            if self.connections:
+                connection_id = list(self.connections.keys())[-1]
+                # TODO: if the most recent connection dies, it should be automatically cleared.
+            else:
+                connection_id = str(uuid4())
 
         if not self.connections.get(connection_id):
             if not self.connect(connection_id=connection_id):