Fix and re-enable `test/node/agent.js` concurrency test

[SgtPooki]

see #83 (comment)

we should implement functionality similar to what libp2p is doing, to ensure requests are prevented from ever making it to the server.

Currently, most of the APIs are using api.post where api is IpfsUtilsHttpClient. We should wrap any of these calls with a queue provider that limits requests based on some settings from users.