routing: use block_in_place() in drop impl

[pronebird]

As described in tokio-rs/tokio#5843, current implementation of Drop in RouteManager would typically lead to panic.

In a multi-threaded runtime the better approach would be to use tokio::task::block_in_place. In essence, even better approach would be to avoid doing async calls from Drop and call stop() somewhere in the clean up routine of a parent scope, but such change would encompass a larger scope and beyond what I am currently looking to fix.

I scanned the code for current_thread_runtime and found none in the way of that Drop, so I think it should work but don't take my word for it.

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This change is 

[dlon]

Thanks. I followed this up with a non-blocking drop impl: #5934