Add a new primitive of a DB-backed channel

The coordinator already had one of these, albeit implemented much worse than
the one now properly introduced. It had to either be sending or receiving,
whereas the new one can do both at the same time.

This replaces said instance and enables pleasant patterns when implementing the
processor/coordinator.

common/db/src/create_db.rs

@@ -29,7 +29,7 @@ pub fn serai_db_key(
 ///
 /// ```ignore
 /// create_db!(
-///   TrubutariesDb {
+///   TributariesDb {
 ///     AttemptsDb: (key_bytes: &[u8], attempt_id: u32) -> u64,
 ///     ExpiredDb: (genesis: [u8; 32]) -> Vec<u8>
 ///   }
@@ -70,3 +70,51 @@ macro_rules! create_db {
     )*
   };
 }
+
+#[macro_export]
+macro_rules! db_channel {
+  ($db_name: ident {
+    $($field_name: ident: ($($arg: ident: $arg_type: ty),*) -> $field_type: ty$(,)?)*
+  }) => {
+    $(
+      create_db! {
+        $db_name {
+          $field_name: ($($arg: $arg_type,)* index: u32) -> $field_type,
+        }
+      }
+
+      impl $field_name {
+        pub fn send(txn: &mut impl DbTxn $(, $arg: $arg_type)*, value: &$field_type) {
+          // Use index 0 to store the amount of messages
+          let messages_sent_key = $field_name::key($($arg),*, 0);
+          let messages_sent = txn.get(&messages_sent_key).map(|counter| {
+            u32::from_le_bytes(counter.try_into().unwrap())
+          }).unwrap_or(0);
+          txn.put(&messages_sent_key, (messages_sent + 1).to_le_bytes());
+
+          // + 2 as index 1 is used for the amount of messages read
+          // Using distinct counters enables send to be called without mutating anything recv may
+          // at the same time
+          let index_to_use = messages_sent + 2;
+
+          $field_name::set(txn, $($arg),*, index_to_use, value);
+        }
+        pub fn try_recv(txn: &mut impl DbTxn $(, $arg: $arg_type)*) -> Option<$field_type> {
+          let messages_recvd_key = $field_name::key($($arg),*, 1);
+          let messages_recvd = txn.get(&messages_recvd_key).map(|counter| {
+            u32::from_le_bytes(counter.try_into().unwrap())
+          }).unwrap_or(0);
+
+          let index_to_read = messages_recvd + 2;
+
+          let res = $field_name::get(txn, $($arg),*, index_to_read);
+          if res.is_some() {
+            $field_name::del(txn, $($arg),*, index_to_read);
+            txn.put(&messages_recvd_key, (messages_recvd + 1).to_le_bytes());
+          }
+          res
+        }
+      }
+    )*
+  };
+}