lib/selection: fix interaction of timeouts with reboots

We use "monotonic" time-stamps for the dead_since field; that breaks on system reboots, in which case we reset the stats. (if the server was categorized as dead)

If the server times out afterwards, we'd fail the condition cur_state.consecutive_timeouts == old_state.consecutive_timeouts so its stats would not update. Therefore we'd get stuck forever in a state where the unusable server has high priority (no_rtt_info).

This commit changes a bit more than was necessary to fix this, including precision of the stats (in some cases).

Fixes #722 (closed)