bump MaxIdleConns(PerHost) like v0.7

[jaffee]

Overview

This helps "connection reset" issues when the cluster is under high query load
from many clients.

This also brings in the default http transport options which were (mistakenly?)
removed by the TLS work. I'm not sure what the ramifications would be of using a
blank http.Transport{} instead of http.DefaultTransport, but it seems best to
avoid a sweeping change.

Fixes #906

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[yuce]

That makes sense. I assumed http.Transport{} would mean the default options, but probably that's not the case.

[jaffee]

yea @yuce - I would have thought so too, but I discovered this in my recent deep dive into net/http - the blank http.Client{} actually uses http.DefaultTransport which has a bunch of options already set.