kad: make `automatic_bootstrap_interval` publicly configurable

[drHuangMHT]

Description

I would like automatic_bootstrap_interval to be made publicly configurable.

Motivation

The default 500ms timeout seems too short, it consumes a lot more resources than before.

Current Implementation

set_automatic_bootstrap_throttle exists but not publicly exported, it's for tests only.

Are you planning to do it yourself in a pull request ?

Yes