Added timeout option to http client

[Irisha]

Optional Timeout for net.Client.

[coveralls]

Coverage decreased (-0.03%) to 75.008% when pulling cdffbe1 on Irisha:patch-1 into f60c514 on yandex:develop.

[direvius]

Is this a finished PR? The Timeout field is not used anywhere, or am I wrong?

[skipor]

Yes, the field is not used yet, we have discussed this in messenger.

[Irisha]

Go’s http package doesn’t specify request timeouts by default, allowing services to hijack your goroutines. Always specify a custom http.Client when connecting to outside services.
https://medium.com/@nate510/don-t-use-go-s-default-http-client-4804cb19f779

I have problem with absence timeout when I fire with keep-alive.