rv1exec reader: implement update ()

Problem: issue flux-framework#991 identified the need for an `rv1exec` implementation of update (). The
need for the implementation is described in detail in the issue, but
the primary motivation is to enable reloading Fluxion when using RV1
without the scheduling key and payload. The reader was not originally
implemented due to the lack of information in the format. Examples
include edges, exclusivity, paths with subsystems, and vertex sizes. To
create a workable implementation, strong assumptions need to be made
about resource exclusivity and edges.

Add support for update () through helper functions that update vertex
and edge metadata and infer exclusivity of node-level resources.