refactor: Make ConnectivityStore use a non-async lock (#7129)

Follow-up to https://github.com/chatmail/core/pull/7125: We now have a
mix of non-async (parking_lot) and async (tokio) Mutexes used for the
connectivity. We can just use non-async Mutexes, because we don't
attempt to hold them over an await point. I also tested that we get a
compiler error if we do try to hold one over an await point (rather than
just deadlocking/blocking the executor on runtime).

Not 100% sure about using the parking_lot rather than std Mutex, because
since https://github.com/rust-lang/rust/issues/93740, parking_lot
doesn't have a lot of advantages anymore. But as long as iroh depends on
it, we might as well use it ourselves.

deltachat-ffi/src/lib.rs

@@ -375,7 +375,7 @@ pub unsafe extern "C" fn dc_get_connectivity(context: *const dc_context_t) -> li
         return 0;
     }
     let ctx = &*context;
-    block_on(ctx.get_connectivity()) as u32 as libc::c_int
+    ctx.get_connectivity() as u32 as libc::c_int
 }
 
 #[no_mangle]