A client per runtime operation made a bulk delete pay a TCP and HTTP/2
handshake for every client. The local runtime now keeps one gRPC client
per API port and replaces it when a core restart moves the port; the
traffic poll keeps its own client, since StatsLastValues is unlocked.
A pooled channel keeps grpc-go's reconnect backoff after the core
restarts, so a call that fails with Unavailable drops the client and
dials again, as a client per call did.
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