Electrum queries are inefficient, may produce server bans
Queries to the electrum server are done in an inefficient way. When using an `electrs` locally hosted back end, the following log messages are produced:
`your wallet uses less efficient method of querying electrs, consider contacting the developer of your wallet. Reason: blockchain.scripthash.get_history called for unsubscribed scripthash: c28a[......]`

This may be related to #797 where the (public) electrum server rejects our requests, probably to avoid DoS attacks.