Allow to construct `ElectrumClient` *without* connecting

[tnull]

Currently, ElectrumClient::new directly tries to establish a connection to the server, i.e., there is no way to successfully construct a client while being offline.

It would be great if there was a way to simply initialize a client, without directly establishing a connection.

This might go hand-in-hand with decoupling the Client object more generally from a single connection, see #165.