node: Refactor how services are stored on the node

[renaynay]

Right now, services are stored as such on the node:

        BlockServ  *block.Service  `optional:"true"`
	ShareServ  share.Service   // not optional
	HeaderServ *header.Service // not optional

Sometimes, the services are nil (for Bridge and Light node, block service will always be nil, for example), while ShareServ is an interface -- it's not uniform.

---

Proposed solution:

Store the lifecycles on the node rather than the actual services themselves as the node doesn't necessarily need to care about its services, just needs to be able to start and stop them (which is done with the fx app anyway at the moment).

[renaynay]

Related to #506

[renaynay]

No longer an issue thanks to turning everything into a Module