fix(runloop) reschedule rebuild timers after running them

[locao]

Summary

Router and plugin iterator rebuild timers were scheduled using ngx.timer.every. If the rebuild processes took longer than worker_state_update_frequency to execute, they would start again while the previous one was still running.

Full changelog

• Change ngx.timer.every calls to ngx.timer.at.
• Previously returned error "plugins iterator was changed while rebuilding it" is now logged as an info message.
• No tests were added, the behavior is the same as before.

Issue reference

There were multiple reports of "plugins iterator was changed while rebuilding it" logged as error.

Fix #8525

FTI-3239