Have sim2h let go of nodes when the connection is lost

[lucksus]

PR summary

Sim2h was not letting go of agents that were long gone, if they were lost because of a connection error. That led to some strange behavior when those agents reconnected again.

This fixes that.

changelog

• if this is a code change that effects some consumer (e.g. zome developers) of holochain core, then it has been added to our between-release changelog with the format

- summary of change [PR#1234](https://github.com/holochain/holochain-rust/pull/1234)

documentation

• this code has been documented according to our docs checklist

[maackle]

👍 if it works!