Add MaxIdleConnsPerHost. Fixes too many open files error.

[emilevauge]

This PR adds an option MaxIdleConnsPerHost to be used when too many open files error are encountered.
As explained here golang/go#6785, during high loads, it seems that:

when the number of requests/s to the proxy goes up, the small amount of bookkeeping that is required to recycle a connection (putIdleConn etc) starts taking just long enough that the next request in the pipeline gets in before a connection is idle. The Transport starts Dial:ing, adding more connections to take care of, and it explodes chaotically. I would prefer if it blocked and awaited an idle connection instead of Dial:ing at some point

Fixes #157 #127

[vdemeester]

LGTM 🐸

[Yggdrasil]

The config file mentions "If zero, DefaultMaxIdleConnsPerHost is used", however I don't see the setting for DefaultMaxIdleConnsPerHost anywhere? In other words, what's the default value?

I just ran into the "Too many open files" problem on one of my servers. I have no idea what to set this to. Is setting MaxIdleConnsPerHost = 200 a good start? How can I tell, please?

[timoreimann]

@Yggdrasil DefaultMaxIdleConnsPerHost is defined in the net/http package: https://golang.org/pkg/net/http/#pkg-constants . It's set to 2.

I will create a PR to add some clarity to the docs.

[timoreimann]

Turns out we use a default of 200 already, but the docs aren't up to date.

#1239 is fixing this.