consumer: reuse lookupd http client

[JieTrancender]

Problem

After some wrong happend on my machine, i found there are many TIME_WAIT status.
I have many nsqd topics and n consumers, consumer making http request which not using Keep-Alive causes this problem.

[[email protected] server]$ netstat -an |grep 19991|awk '{print $6}'|sort|uniq -c
      2 ESTABLISHED
      1 LISTEN
   1401 TIME_WAIT

Solution

1. using go http.Transport features(keepalives,timeouts,dualstack)
2. reuse http client

[[email protected] server]$ netstat -an |grep 19991|awk '{print $6}'|sort|uniq -c
   1217 ESTABLISHED
      1 LISTEN
    305 TIME_WAIT

3. support setting lookupd http client

[[email protected] server]$ netstat -an |grep 19991|awk '{print $6}'|sort|uniq -c
    131 ESTABLISHED
      1 LISTEN
    355 TIME_WAIT

[JieTrancender]

@ploxiln Thank you for your review, i have finished these modifications, please review it again.

[ploxiln]

you can leave this as it was; it's still true that NewDeadlineTransport was un-exported (and then now has been un-existed ;)

[ploxiln]

If you undo this little change to UPGRADING, and squash down to one commit (with cleaned-up commit message), I'll merge.

[ploxiln]

This seems redundant with the http.Client Timeout but I guess it's fine.
(Dialer Timeout above could probably be a shorter default, probably matching the 10s TLSHandshakeTimeout below. But again, I guess it's fine, doesn't really hurt.)

[JieTrancender]

emmm, can request a review to other people? @ploxiln