Describe the enhancement

After a call with @LLFourn, the follow things will be added (either to this PR, or a separate PR):

1. The PersistBackend<T, C> trait currently has a generic T for the "in memory representation". This also constrains C to "accompany" T, thus requiring us to have something like Tracker in Implement persistence with the new structures #965. The "onioning" of these structures is unfortunate. However, there is a way to do PersistBackend without T and just rely on C! Let's experiment with that to see if it cleans up the API and reduces "onioning".
   -- Completed by Implement persistence with the new structures #965

2. ChainOracle::get_chain_tip(&self) -> Result<Option<BlockId>, Self::Error> will be added. This is needed as we need something to input as the "static block" for the is_block_in_best_chain call. Additionally, a structure that is capable of implementing ChainOracle functionality should also have the functionality to return the tip of the best chain.

3. OwnedIndexer is bad. Here is something better:

   pub trait OwnedTxOuts {
       type SpkIndex;
       type Iter: Iterator<Item = (Self::SpkIndex, OutPoint)>;
       fn owned_txouts(&self) -> Self::Iter;
   }

   This thing iterates over all outpoints that we own (alongside the spk index we give it). To get the owned UTXO set, we filter over this. Same thing for balance.
   -- Turns out OwnedTxOuts is not needed either, all removed in Improve txout listing and balance APIs for redesigned structures #975