Timeouts slip if there are no yields for a long time

Suppose current fiber does not yield for a time T1. If it arms a timer using ev_timer to trigger in time T2 then timer will trigger in T2 - T1 time instead of T2 in the best case (fiber yields right after arming the timer and there are no other fibers). The issue is ev updates its current time (mn_now) only when when it grabs control back. So in the above situation at the time of arming the current time is T1 behind "real time".

There is ev_now_update to update ev current time at any moment.

Idea: call ev_now_update in low resolution timer.

The slip was observed on attempt to add debug ASAN workflow to CI in PR https://github.com/tarantool/tarantool-ee/pull/572. The time slip for debug ASAN build is about 0.3 sec at the moment Tarantool run etcd configuration code on startup. This caused etcd-client/config_etcd_test.lua test to fail because the timeout for reply from etcd is set to 0.3 sec too and time slip cause the timeout to trigger immediately.