Previously, the functions and subclasses in gpcrondump, gpdbrestore, dump.py,
and restore.py all required long lists of arguments to be passed around and
required many helper functions.  This made adding new command-line options
or new features quite time-consuming to implement and to test.

Now, a Context class has been added to backup_utils.py that holds all of the
variables used for command-line options, as well as other variables used in
many places during backup and restore; additionally, many helper functions
have been condensed into a few functions within Context, and in the future
more such functions can be moved to Context as needed.  This will make it
much easier to expand dump and restore functionality going forward.