Fix of pool leaking by TCP-keepalive

[NumminorihSF]

There is some bug with connection pool.
To reproduce it - you need connect to postgresql, start request and kill db process by kill -9. Client will not be free on any time.

To fix this, we can add heartbeat to client connection (#913) or add TCP-keepalive.

I have tested it. On TCP-socket close it return error to client#query callback function. I think, that it is the best resolve of this problem.

P.S. Please, if you will merge one of my PR - close another one.

[brianc]

@NumminorihSF sorry for missing this! This definitely looks like a solid approach. I'll go ahead & merge this & close the other pull request! In order to make this backwards compatible I'm going to add this as a config option to the client:

var client = new Client({
  keepAlive: true
})

It will default to false and only alter current behavior if set to true.