[REQ] Use send_timeout() instead of send() to prevent deadlocks

[mhatzl]

Note: This cannot be implemented at the moment, because std::mpsc::SyncSender has no public send_timeout() function.

Requires send_timeout() to become public.
Private impl see: https://github.com/rust-lang/rust/blob/f8276c94ac06b272e88fb1bb9c5f6615fc5876ef/library/std/src/sync/mpmc/mod.rs