`OnceCell` doesn't call `drop`, which makes the spawned
`bitcoind`/`electrsd` instances linger around after our tests have
finished. To fix this, we move them out of `OnceCell` and let every test
that needs them spawn their own instances. This additional let us drop
the `OnceCell` dev dependency.
To support HTTPS endpoints, the async HTTP library `reqwest` needs one of
the `-tls` features enabled. While the users could specify this in their
own cargo dependencies, we here provide a new `esplora-async-https`
feature for conveinience.
