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 changes how the context is created and inherited to better
reflex the relationship of the components. We now have the following
context structure,
```
For ephemeral nodes, the context chain is,
main HarnessTest -> sub HarnessTest -> HarnessNode -> HarnessRPC
For standby nodes, the context chain is,
main HarnessTest -> standby HarnessNode -> HarnessRPC
```
This commit adds a new struct, `HarnessNode`, to manage the lnd process
used in our itest. This struct is built upon `HarnessRPC`, `State`, and
`NodeWatcher`.
