When spawning using `Handle` while on the executor, tasks were being
double counted. This prevented the number of active tasks to reach zero,
thus preventing the executor from shutting down.
This changes `spawn` to check if being called from the executor
**before** incrementing the number of active tasks.
Fixes#760
The futures 0.2 crate is not intended for widespread usage. Also, the
futures team is exploring the compat shim route.
If futures 0.3 support is added to Tokio 0.1, then a different
integration route will be explored, making the current code unhelpful.
Extract `tokio::executor::current_thread` to a tokio-current-thread
crate. Deprecated fns stay in the old location. The new crate only
contains thee most recent API.
