litep2p/discovery: Fix memory leak in `litep2p.public_addresses()` (#5998)

This PR ensures that the `litep2p.public_addresses()` never grows
indefinitely.
- effectively fixes subtle memory leaks
- fixes authority DHT records being dropped due to size limits being
exceeded
- provides a healthier subset of public addresses to the `/identify`
protocol

This PR adds a new `ExternalAddressExpired` event to the
litep2p/discovery process.

Substrate uses an LRU `address_confirmations` bounded to 32 address
entries.
The oldest entry is propagated via the `ExternalAddressExpired` event
when a new address is added to the list (if capacity is exceeded).

The expired address is then removed from the
`litep2p.public_addresses()`, effectively limiting its size to 32
entries (the size of `address_confirmations` LRU).

cc @paritytech/networking @alexggh



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Signed-off-by: Alexandru Vasile <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Dmitry Markin <[email protected]>