Optionally overwrite timeout for Logcache::Client

[philippthun]

The timeout used for the Logcache GRPC client was hard-coded to 250 seconds. With this change the timeout can be (optionally) configured to a different value.

Running "cf push" results in requests to /v3/processes/:guid/stats. But in case Logcache is unavailable, "cf push" can actually continue/succeed without showing the requested data. The default timeout resulted in long-running requests without adding much value.

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