Ignoring maxSockets

It seems maxSockets agent option is ignored when HttpProxyAgent is used.

```javascript
const got = require('got');
const { HttpProxyAgent } = require('hpagent')
const http = require('http')

const opts = {
maxSockets: 1,
proxy: 'http://localhost:5555'
};
const hpagent = new HttpProxyAgent(opts);

const start = new Date();
const elapsed = () => { return (new Date().getTime() - start.getTime()) / 1000.0 }

// They run parallel on 2 different sockets, ignoring maxSockets
got('http://httpbin.org/delay/3', {
agent: {
http: hpagent
}
}).then(() => console.log(`hpagent Request1 done in : ${elapsed()}`));

got('http://httpbin.org/delay/3', {
agent: {
http: hpagent
}
}).then(() => console.log(`hpagent Request2 done in : ${elapsed()}`));

const agent = new http.Agent(opts);
// They run sequentially as maxSockets is 1
got('http://httpbin.org/delay/4', {
agent: {
http: agent
}
}).then(() => console.log(`http.agent Request1 done in : ${elapsed()}`));

got('http://httpbin.org/delay/4', {
agent: {
http: agent
}
}).then(() => console.log(`http.agent Request2 done in : ${elapsed()}`));

```

Console output is :
```
hpagent Request1 done in : 3.414
hpagent Request2 done in : 3.631
http.agent Request1 done in : 4.413
http.agent Request2 done in : 8.607
```
As you see, in contrast to http.Agent, hpagent sends second request immediately, without waiting for first socket to finish.

Reproduce the issue with the provided got, HttpProxyAgent, and http.Agent example, then inspect how HttpProxyAgent handles maxSockets compared with Node's http.Agent. Done means two delayed requests using one HttpProxyAgent socket run sequentially when maxSockets is 1, matching the demonstrated http.Agent behavior.

Written by the indexing model from the issue text.