Make COPY tolerant to connection timeouts if connections exist

We sometimes see connection timeouts during COPY, which causes the whole COPY to abort. However, if we already have an active connection to the worker node we could reuse that connection instead of establishing a new one.

Start by tracing the COPY path that establishes connections to worker nodes, then inspect how existing active connections are tracked and selected. Reproduce or locate coverage for a connection timeout during COPY; done means an available active connection is reused instead of aborting the COPY.

Written by the indexing model from the issue text.