This commit changes how a node's state is created during startup. When
it already has states, only its internal states are reset since these
info are only useful for the finished test. The name `InitRPCClients` is
changed to `Initialize` to reflect the fact that it's more than
initializing RPC clients.
This commit adds a new package `lntemp/node` and inside it a new struct
`State` is added to track the internal node state. This enables us using
the same running nodes accross tests without wrong uncleaned states.