Use async-native esplora backend

[thomaseizinger]

At the moment, we are using the https://github.com/bitcoindevkit/rust-electrum-client as the backend for BDK.

BDK also has a reqwest-based esplora backend: https://github.com/bitcoindevkit/bdk/blob/master/src/blockchain/esplora/reqwest.rs

We already depend on reqwest, meaning doing this would cut down our dependency tree and compile times. Additionally, this one is using async and thus provides a better integration with our codebase. the electrum-client is using its own background thread IIRC.

Open problems:

• We are using the same client for monitoring and the implementation of EsploraBlockchain is completely hidden behind a trait. Thus we cannot access the necessary functionality that we need for monitoring.

[thomaseizinger]

Reasons why this is tech-debt:

• The electrum client we are currently using is blocking which can cause all sorts of weirdness in an async system.
• Swapping to BDK's esplora client is non-trivial because their futures don't implement Sync Futures returned in library are not send bitcoindevkit/bdk#165

[bonomat]

This project is unmaintained