litep2p: Increase keep-alive to 10 seconds to mirror libp2p (#7488)

This PR ensures that litep2p will keep an idle connection alive for 10
seconds.

The bump from 5 seconds is done to mirror the libp2p behavior and
potentially improve connection stability:

https://github.com/paritytech/polkadot-sdk/blob/a07fb323

/substrate/client/network/src/service.rs#L542-L549

cc @paritytech/networking

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Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>