sd-dhcp-client: initialize event source and so on in client_start_delayed()

sd-dhcp-client: initialize event source and so on in client_start_delayed()

When we start the client, any previous state/configuration should be cleaned.
Let's effectively do the same thing as client_initialize() in that
function.

This also several assertions in client_start_delayed() to
sd_dhcp_client_start(). These kind of checks should be done earlier.