snapshot: cleanup/document much of fork-based snapshotting

We can consolidate much of the fork-based snapshotting "context"
variables so that they are easier to understand; we essentially only
need two types of contexts, the "shared" context used for the client
program (i.e., the model-checker) and the "private" context, which
each snapshot process uses for running and managing its state. That way,
we only have a single process using the "shared" context at a time,
while the others reside in their private context.

Additionally, I add an "exit" context, which is used just to help with
terminating processes once they have to roll back.

Otherwise, this commit contains renaming and documenting of several
fields, variables, etc.