Worker client replacement

What was changed

Added Worker::replace_client. Notes:

Replacement unregisters itself with old client and re-registers with new client for eager workflow start
Uses lock of arc, so no currently operating poll on existing client will be interrupted (this has been tested)
This is as close as we can reasonably get to dynamic client endpoints without working through Tonic's connectivity
- This is actually much cleaner for SDKs since they only really care about this for workers and can't interrupt anything running anyways
Had to change a few internal things but nothing drastic
Wrote test that starts two servers and swaps between (had to lower polling timeout on them)