rust-esplora-client shouldn't introduce an async runtime #102
Description
In #98, a dependency on the async_std runtime was introduced. IIUC, this was done only to use its sleep method.
I don't think rust-esplora-client should make choices regarding which async runtime users should use. If anything, it should be using the by now default tokio rather than async_std.
I believe I already brought this up in the discussion of the precursor #71, which however seems now gone for some reason.
An alternative to pulling in an extra dependency for this would simply be to have the user specify a sleep callback function that would use whatever sleep functionality they have available in their runtime.