The node.NewNode method is pretty complex at the moment, an in order to address issues like #3156, we need to simplify the interface for partial node instantiation. In some places, we don't need to build up a full node (like in the node.TestCreateProposalBlock test), but the complexity of such partial instantiation needs to be reduced.

This PR aims to eventually make this easier/simpler.

See also this gist https://gist.github.com/thanethomson/56e1640d057a26186e38ad678a1d114c for some background work done when starting to refactor here.

## Commits:

* [WIP] Refactor node.NewNode to simplify

The `node.NewNode` method is pretty complex at the moment, an in order
to address issues like #3156, we need to simplify the interface for
partial node instantiation. In some places, we don't need to build up a
full node (like in the `node.TestCreateProposalBlock` test), but the
complexity of such partial instantiation needs to be reduced.

This PR aims to eventually make this easier/simpler.

* Refactor state loading and genesis doc provider into state package

* Refactor for clarity of return parameters

* Fix incorrect capitalization of error messages

* Simplify extracted functions' names

* Document optionally-prefixed functions

* Refactor optionallyFastSync for clarity of separation of concerns

* Restructure function for early return

* Restructure function for early return

* Remove dependence on deprecated panic functions

* refactor code a bit more

plus, expose PEXReactor on node

* align logger names

* add a changelog entry

* align logger names 2

* add a note about PEXReactor returning nil