Track and amortize operating costs to ensure solvency

Implements most of #297 to the point I'm fine closing it. The solution
implemented is distinct than originally designed, yet much simpler.

Since we have a fully-linear view of created transactions, we don't have to
per-output track operating costs incurred by that output. We can track it
across the entire Serai system, without hooking into the Eventuality system.

Also updates documentation.

processor/src/multisigs/scheduler.rs

@@ -322,8 +322,6 @@ impl<N: Network> Scheduler<N> {
     }
 
     for chunk in utxo_chunks.drain(..) {
-      // TODO: While payments have their TXs' fees deducted from themselves, that doesn't hold here
-      // We need the documented, but not yet implemented, virtual amount scheme to solve this
       log::debug!("aggregating a chunk of {} inputs", N::MAX_INPUTS);
       plans.push(Plan {
         key: self.key,