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.

Inizia riproducendo il confronto segnalato tra loop.time() di uvloop e vanilla asyncio su Linux usando Python 3.6, quindi traccia l'implementazione dell'orologio del loop degli eventi in uvloop. Il lavoro è completo quando operazioni rapide successive riportano tempi dell'ordine dei microsecondi comparabili a quelli del loop degli eventi della stdlib, con un test di regressione che copra questo comportamento.