client: Use single time variable when handling heartbeat response.

[jrasell]

When the client handles an update status response from the server, it modifies its heartbeat stop tracker with a time set once the RPC call returns. It optionally also emits a log message, if the client suspects it has missed a heartbeat.

These times were originally tracked by two different calls to the time function which were executed 2 microseconds apart. There is no reason we cannot use a single time variable for both uses which saves us one whole call to time.Now.

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[tgross]

LGTM!