Remove a peer's live routes when its session goes down (closes #56)

RIS sends no withdrawals for the routes of a peer whose BGP session goes
down, so those routes stayed in the live tables for good.

The streamer now hands STATE messages to the handlers that want them. When
the prefix handler gets one whose state is down, it drops the peer's
updates still in its batch and notes the peer; the next flush deletes all
of the peer's live routes, IPv4 and IPv6, before writing the batch, so
updates the peer sends after the down are kept. The new store method keeps
the in-memory route counts and prefix distribution exact, and an index on
peer_ip lets it find the peer's routes without reading the whole table.

Model: opus-5-5

README.md

@@ -128,7 +128,10 @@ routewatch/
 4. **Handler Details**
    - **ASHandler**: Tracks all ASNs seen in AS paths
    - **PeerHandler**: Records BGP peer information
-   - **PrefixHandler**: Maintains live routing table (upserts on announcement, deletes on withdrawal)
+   - **PrefixHandler**: Maintains live routing table (upserts on announcement, deletes on withdrawal,
+     deletes all of a peer's routes when its BGP session goes down). Routes are kept per peer IP, so
+     when one peer IP has sessions on two collectors, a session going down on one also removes the
+     routes the other still carries, until they are announced again.
    - **PeeringHandler**: Extracts AS peering relationships from AS paths
 
 5. **HTTP Request Flow**