Uvloop internal clock is not microsecond accurate

• uvloop version: 0.14, master
• Python version: 3.6
• Platform: Linux
• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?: yes
• Does uvloop behave differently from vanilla asyncio? How?: loop.time() reports time with the accuracy of milliseconds on uvloop, microseconds (or greater) on the stdlib event loop.

Today I debugged some failing tests in the httpcore project and it turns out they were relying on time passing between two subsequent operations. The code was running so fast, however, that the returned clock value on uvloop was the same as it was before, due to the lesser accuracy. This should hopefully be easy to fix.

首先，在使用 Python 3.6 的 Linux 上重现报告中 uvloop 的 loop.time() 与 vanilla asyncio 的比较，然后追踪 uvloop 中事件循环时钟的实现。当连续的快速操作报告出与 stdlib 事件循环相当的微秒级计时，并且有一个覆盖该行为的回归测试时，即视为完成。