dbus: Use ThreadPool instead of zbus blocking API

The `zbus` blocking API just wraps the async API with `block_on`, so we
may as well use our own executor.

Cargo.toml

@@ -88,6 +88,7 @@ clap_lex = "0.7"
 parking_lot = "0.12.5"
 logind-zbus = { version = "5.3.2", optional = true }
 futures-executor = { version = "0.3.31", features = ["thread-pool"] }
+futures-util = "0.3.31"
 
 [dependencies.id_tree]
 branch = "feature/copy_clone"