[blockchain] Add traits to reuse Blockchains across multiple wallets

Add two new traits:
- `StatelessBlockchain` is used to tag `Blockchain`s that don't have any
  wallet-specic state, i.e. they can be used as-is to sync multiple wallets.
- `BlockchainFactory` is a trait for objects that can build multiple
  blockchains for different descriptors. It's implemented automatically
  for every `Arc<T>` where `T` is a `StatelessBlockchain`. This allows a
  piece of code that deals with multiple sub-wallets to just get a
  `&B: BlockchainFactory` to sync all of them.

These new traits have been implemented for Electrum, Esplora and RPC
(the first two being stateless and the latter having a dedicated
`RpcBlockchainFactory` struct). It hasn't been implemented on the CBF
blockchain, because I don't think it would work in its current form
(it throws away old block filters, so it's hard to go back and rescan).

This is the first step for #549, as BIP47 needs to sync many different
descriptors internally.

It's also very useful for #486.

src/blockchain/esplora/ureq.rs

@@ -98,6 +98,8 @@ impl Blockchain for EsploraBlockchain {
     }
 }
 
+impl StatelessBlockchain for EsploraBlockchain {}
+
 impl GetHeight for EsploraBlockchain {
     fn get_height(&self) -> Result<u32, Error> {
         Ok(self.url_client._get_height()?)