Clear sockets perform worse than TLS socket in some cases

• Version: v13.0.0-pre
• Platform: Linux tufopad 4.15.0-48-generic #51-Ubuntu SMP Wed Apr 3 08:28:49 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
• Subsystem: net

While writing and performing tests for this issue opened by @alexfernandez and fixed by @jmendeth, I noticed that in some cases, clear connections seem to perform worse than TLS connections, specially after the fix that enhances significantly the performance of the TLS connections. I will patch the existing secure-pair benchmark test to expose the issue.

The performance of clear connections seems to be the same across node versions (I've tested it in node 8, 10 and 13-pre). This means that this issue is not a regression, but an enhancement, and the fix needed is probably very similar to the one that solved the secure-pair performance issue.

从 issue 中提到的现有 secure-pair 基准测试开始，比较其中的 clear 和 TLS 用例；查看 issue #20263 和 PR #27861，了解相关的性能变更。完成的标准是：基准测试能够暴露 clear socket 的性能下降，并且 net 实现得到改进，使 clear 连接在报告的用例中不再表现不佳。