Allow using initialized connext client

[kilrau]

Some users may want to generate seeds on their own and initialize swap clients like lnd and connext manually with differing seeds.

As this use case already works with lnd, xud should check if the connext client is initialized (hit any endpoint to see if it's initialized -> it will error if not) and use the client as is without generating a child seed and seed parameter in /connect.

[kilrau]

Related: #1929