Reduce `NewExternalAddrCandidate` reports from `identify`

[drHuangMHT]

Description

Report NewExternalAddrCandidate per instance, e.g. one instance of identify behaviour should only report external address candidate only once unless expired.

Motivation

NewExternalAddrCandidate event will not flood the event handling system during DHT walk-through.

Current Implementation

Currently NewExternalAddrCandidate is reported per connection, e.g. the same connection won't report already reported addresses but new connection will. So when bootstraping a DHT network NewExternalAddrCandidate will flood in.

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

Yes