Allow scheduler's creation of transactions to be async and error

I don't love this, but it's the only way to select decoys without using a local
database. While the prior commit added such a databse, the performance of it
presumably wasn't viable, and while TODOs marked the needed improvements, it
was still messy with an immense scope re: any auditing.

The relevant scheduler functions now take `&self` (intentional, as all
mutations should be via the `&mut impl DbTxn` passed). The calls to `&self` are
expected to be completely deterministic (as usual).

processor/monero/src/rpc.rs

@@ -5,7 +5,6 @@ use monero_simple_request_rpc::SimpleRequestRpc;
 
 use serai_client::primitives::{NetworkId, Coin, Amount};
 
-use serai_db::Db;
 use scanner::ScannerFeed;
 use signers::TransactionPublisher;
 
@@ -15,12 +14,11 @@ use crate::{
 };
 
 #[derive(Clone)]
-pub(crate) struct Rpc<D: Db> {
-  pub(crate) db: D,
+pub(crate) struct Rpc {
   pub(crate) rpc: SimpleRequestRpc,
 }
 
-impl<D: Db> ScannerFeed for Rpc<D> {
+impl ScannerFeed for Rpc {
   const NETWORK: NetworkId = NetworkId::Monero;
   // Outputs aren't spendable until 10 blocks later due to the 10-block lock
   // Since we assumed scanned outputs are spendable, that sets a minimum confirmation depth of 10
@@ -39,15 +37,16 @@ impl<D: Db> ScannerFeed for Rpc<D> {
     &self,
   ) -> impl Send + Future<Output = Result<u64, Self::EphemeralError>> {
     async move {
-      // The decoys task only indexes finalized blocks
-      crate::decoys::NextToIndexBlock::get(&self.db)
-        .ok_or_else(|| {
-          RpcError::InternalError("decoys task hasn't indexed any blocks yet".to_string())
-        })?
-        .checked_sub(1)
-        .ok_or_else(|| {
-          RpcError::InternalError("only the genesis block has been indexed".to_string())
-        })
+      Ok(
+        self
+          .rpc
+          .get_height()
+          .await?
+          .checked_sub(1)
+          .expect("connected to an invalid Monero RPC")
+          .try_into()
+          .unwrap(),
+      )
     }
   }
 
@@ -128,7 +127,7 @@ impl<D: Db> ScannerFeed for Rpc<D> {
   }
 }
 
-impl<D: Db> TransactionPublisher<Transaction> for Rpc<D> {
+impl TransactionPublisher<Transaction> for Rpc {
   type EphemeralError = RpcError;
 
   fn publish(