Does not support specifying max-connections

I believe it can be specified in creds like this:

``` clojure
(assoc cred :client-config {:max-connections 100 :user-agent "Amazonica"})
```

https://github.com/mcohen01/amazonica/blob/master/test/amazonica/test/s3.clj

The default is 50. Not sure about the absolute max limit set by S3.
