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.

まず、Python 3.6を使用するLinux上で、uvloopのloop.time()とvanilla asyncioの報告された比較を再現し、その後uvloopにおけるイベントループのクロック実装を追跡します。連続する高速な操作で、stdlibのイベントループと同等のマイクロ秒レベルのタイミングが報告され、この動作をカバーするリグレッションテストがあれば完了です。