timesync: increase retry interval more slowly

The new retry intervals are [15, 20, 26, 34, 45, 60, 80, 106, 141, 188, 250,
333, 360, ...]. This should allow graceful response if a transient network
failure is encountered. Growth is exponential, but with a small power and
capped to a non-too-large value so that we resynchronize within a few minutes
after network is restored. I made the minimum 15 s to make sure that we never
send packets more often than that.

Fixes #16492.