swim: do not use ev_timer_start

Appeared that libev changes 'ev_timer.at' field to a remaining
time value, and it can't be used as a storage for a timeout. By
the same reason ev_timer_start() can't be used to reuse a timer.

On the contrary, 'ev_timer.repeat' is not touched by libev, and
ev_timer_again() allows to reuse a timer.

This patch replaces 'at' with 'repeat' and ev_timer_start() with
ev_timer_again().

The bug was not detected by unit tests, because they implement
their own event loop and do not change ev_timer.at. Now they do
to prevent a regression.

Part of #3234