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/main.rs

@@ -16,7 +16,6 @@ use crate::key_gen::KeyGenParams;
 mod rpc;
 use rpc::Rpc;
 
-mod decoys;
 /*
 mod scheduler;
 use scheduler::Scheduler;