refactor: move async runtime to application module

To have a clear distinction between code dealing with OS process
characteristics and code of ncspot itself, it makes sense to move the
async runtime together with ncspot as it doesn't have anything to do
with the OS process.

src/main.rs

@@ -59,14 +59,8 @@ fn register_backtrace_panic_handler() {
     }));
 }
 
-lazy_static!(
-    /// The global Tokio runtime for running asynchronous tasks.
-    static ref ASYNC_RUNTIME: tokio::runtime::Runtime = tokio::runtime::Builder::new_multi_thread()
-        .enable_all()
-        .build()
-        .unwrap();
-);
-
+// Functionality related to the operating system process itself is implemented here.
+// Functionality related to ncspot goes into `Application`.
 fn main() -> Result<(), String> {
     register_backtrace_panic_handler();