Make progres on handling NewSet events

Further bones out the coordinator.

coordinator/src/db.rs

@@ -0,0 +1,25 @@
+pub use serai_db::*;
+
+#[derive(Debug)]
+pub struct MainDb<D: Db>(pub D);
+impl<D: Db> MainDb<D> {
+  pub fn new(db: D) -> Self {
+    Self(db)
+  }
+
+  fn main_key(dst: &'static [u8], key: impl AsRef<[u8]>) -> Vec<u8> {
+    D::key(b"MAIN", dst, key)
+  }
+
+  fn event_key(id: &[u8], index: u32) -> Vec<u8> {
+    Self::main_key(b"event", [id, index.to_le_bytes().as_ref()].concat())
+  }
+  pub fn handle_event(&mut self, id: [u8; 32], index: u32) {
+    let mut txn = self.0.txn();
+    txn.put(Self::event_key(&id, index), []);
+    txn.commit();
+  }
+  pub fn handled_event(&self, id: [u8; 32], index: u32) -> bool {
+    self.0.get(Self::event_key(&id, index)).is_some()
+  }
+}