Discovering doesn't reflect what tnt router discovering does

Original: https://github.com/KaymeKaydex/go-vshard-router/issues/121

Discovering doesn't reflect what tnt router discovering does.
Maybe, we should repeat it's behavior?
See https://github.com/tarantool/vshard/blob/e1c806e1d3d2ce8a4e6b4d498c09051bf34ab92a/vshard/router/init.lua#L296