Add UpdateConnection to endpoint

[DimitrisJim]

Summary

Would be nice to add a helper function on Endpoint that takes care of setting connection fields as needed, see relevant issue #3984 for channels. SImilar to that, we could have an UpdateConnection function that handles updating connection fields.

func (endpoint *Endpoint) UpdateConnection(updater func(connection *connectiontypes.ConnectionEnd)) error {
	connection := endpoint.GetConnection()

	updater(&connection)
	endpoint.SetConnection(connection)

	endpoint.Chain.Coordinator.CommitBlock(endpoint.Chain)

	return endpoint.Counterparty.UpdateClient()
}

Adding this function allows SetConnection/GetConnection to be private or, possibly, inlined into UpdateConnection.

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[DimitrisJim]

Would be best to implement and refactor relevant usages after 04-channel-ugprades has been merged.