add TCP keep-alive to client side

[sigxcpu76]

No description provided.

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[googlebot]

CLAs look good, thanks!

[sigxcpu76]

This patches the client-side TCP connection with keep-alive (60 seconds interval) as a fix for #147

[hfwang]

Sorry for the long delay. The change looks good!

I'm going to pull this into the Google internal repository, and test it internally for this week, assuming all goes well, I'll merge this!

[robsonpeixoto]

News @hfwang ?

[hfwang]

Some comments were made when this was reviewed internally, I've removed the extra log line and the call to SetLinger. Over to easwars for review!