Right now no `http.Agent` is used for requests made using the http
client in node, which means each request opens a new connection which
can end up hitting process resource limits which means connections get
dropped.

The change here sets a default `http.Agent` with a `keepAlive: true` and
`maxSockets` of 6 which is consistent with [browsers](https://tools.ietf.org/html/rfc2616#section-8.1.4)
and [native apps](https://developer.apple.com/documentation/foundation/nsurlsessionconfiguration/1407597-httpmaximumconnectionsperhost?language=objc).

The user can override the agent passed to the ipfs-http-client constructor
to restore the previous functionality:

```
const http = require('http')
const createClient = require('ipfs-http-client')

const client = createClient({
  url: 'http://127.0.0.1:5002',
  agent: new http.Agent({
    keepAlive: false,
    maxSockets: Infinity
  })
})
```

Refs: #3464