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.

Comienza reproduciendo la comparación reportada de loop.time() de uvloop con vanilla asyncio en Linux usando Python 3.6 y, después, rastrea la implementación del reloj del bucle de eventos en uvloop. El trabajo estará terminado cuando operaciones rápidas sucesivas informen de tiempos con precisión de microsegundos comparables a los del bucle de eventos de la stdlib, con una prueba de regresión que cubra este comportamiento.