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.

Commencez par reproduire la comparaison signalée entre loop.time() de uvloop et vanilla asyncio sous Linux avec Python 3.6, puis examinez l'implémentation de l'horloge de la boucle d'événements dans uvloop. Le travail est terminé lorsque des opérations rapides successives indiquent des temps de l'ordre de la microseconde, comparables à ceux de la boucle d'événements de la stdlib, avec un test de régression couvrant ce comportement.