Fixes/3649 use same http agent for requests

[gravityvi]

Changes

• use a single instance of HTTP agent for requests

Impacts

• fixed keep_alive not getting honoured  #3649

[github-actions]

Status

• ❌ No modified files found in the types directory.
  Please make sure to include types for any changes you have made. Thank you!.

[swrdfish]

Looks good

[beatfactor]

I was suggesting to refactor this into something like

function getAgent(secure) {
  const protocol = secure ? https : http;
  return __httpKeepAliveAgent__ = new protocol.Agent({
      keepAlive: true,
      keepAliveMsecs,
      maxSockets
    });
}

[gravityvi]

Refactored and removed __httpsKeepAliveAgent__ as we would need only a variable per test session/process (either http or https agent). For multiple envs we use worker_threads or child process which won't share instances of global variables.