Add agent socket reuse

[yassernasc]

Not included on this first draft:

• request pool (maxSockets, maxTotalSockets and maxTotalSockets aren't being used yet)
• 'agentRemove' event
• agent.reuseSocket request parameter added
• scheduling option
• agent timeout support

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What is included:

• agent.keepSocketAlive
• agent.getName (minimal version)
• socket reuse with agent.getSocket and socket 'free' event
• agent.destroy

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Extra thing: because http.request by default uses the global agent, which enables keepAlive by default, it made some previous tests start to get stuck, the solution was to add agent: false option.

[kasperisager]

Solid start! 👌 The tests stalling without agent: false smells like a missing unref() somewhere so that's worth looking into; they should pass just fine even with the global agent.

[yassernasc]

The tests stalling without agent: false smells like a missing unref() somewhere so that's worth looking into; they should pass just fine even with the global agent.

Maybe it's missing somewhere at bare-tcp package layer because the unref seems to be called only at the agent.keepSocketAlive on nodejs agent implementation.

[kasperisager]

Looking into the HTTP server implementation in Node.js, it explicitly closes idle connections when closing the server: https://github.com/nodejs/node/blob/362afa52ebe462a39874915e5e70d261db153c58/lib/_http_server.js#L592-L606. The lack of that is most likely what causes the tests to stall.

[yassernasc]

I had a hard time making the CI work after the _closeConnections implementation, the only test that still needs agent: false is the server and client do big writes because it hangs on linux, the request ‘close’ event doesn't seem to be triggered;

At server._closeConnections, I used socket.end instead socket.destroy for some problems with tests too.

[kasperisager]

I tweaked the last bits so this should be ready to land now 👏