Remove borrow on `node.send_output` for python

[haixuanTao]

This PR removes a borrow on send that should make it less adverse to race conditions.

This however still has some issues when concurrently receiving and sending.

See: #746

[phil-opp]

This seems to include commits of other PRs

[phil-opp]

make it less adverse to race conditions.

This is a bit misleading, the race condition is that concurrent operations are not allowed right now. This PR removes that limitation. Perhaps you could clarify this in the PR description?