solver: simplify edge-related functions in the solver

Many of the functions in the solver that operated on the edges were very
large which obscured their usage and the overall flow of the scheduler.
This change refactors those functions into separate smaller functions to
make it easier to follow the overall flow of the scheduler.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

Author: jsternberg