raft: fix ev_timer.at incorrect usage

ev_timer.at was used as timeout. But after ev_timer_start() it
turns into the deadline - totally different value.

The patch makes sure ev_timer.at is not used in raft at all.

To test that the fakeev subsystem is patched to start its time not
from 0. Otherwise ev_timer.at often really matched the timeout
even for an active timer.

(cherry picked from commit e51c61ae)