Work to be done as part of #1290.

Context

#1286 answers the question "Does a Node need an in-memory copy of the genesis for its entire lifecycle?" with a No.

After initialization, a Node only requires the GenesisTime field from the genesis. Once a Node has completed its startup, which includes initializing an Environment struct serving the RPC APIs is initialized, it no longer needs the genesis at all.

ToDo

• Change the implementation of Node so that it no longer keeps a pointer to the GenesisDoc after startup.
• Node stores the GenesisTime as a field to use it after startup.