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`.