use uv_now() to get consistent time when running netmgr callbacks

The following discussion from !7937 (merged) should be addressed:

As we are working with libuv timeouts, it would be better to use uv_now() (perhaps disguised as isc_nmhandle_now() instead of isc_time_now() - that way you would not have to worry about consistent value - that would be provided by libuv.

However, as we need to backport this to 9.18, such changes should be a separate commit or even separate MR.

isc_nmhandle_now() (or maybe isc_loop_now()) sounds like a good idea but should be a different issue.