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.
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