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 replaces the keys used in the maps `activeNodes` and
`standbyNodes` from `pubkeyStr` to `NodeID` such that even the node is
not properly started, we can still clean it up during shutdown.
This commit adds a new struct, `nodeManager`, to manage running nodes.
It keeps track of the running nodes, and manages the start and stop of
the nodes with the context of `HarnessTest`.
