feat: add ConnectionManager#getAll

[wemeetagain]

The implementation is almost identical to ConnectionManager#get except instead of returning the first open connection or null, getAll returns an array of all open connections (or an empty array).

Perhaps get could be refactored to use getAll to avoid code duplication?

Open to any suggestions, naming, implementation, otherwise

See ChainSafe/js-libp2p-gossipsub#83 (comment)

[vasco-santos]

As you suggest, I think that get could use getAll internally

[jacobheun]

This sounds good to me, switching get over to leverage getAll would be great.