Reduce memory usage

Today GAM can use a lot of memory when running a command like:

gam report user

in a large domain. Any command that uses `gapi.get_all_pages()` may use a LOT of memory while downloading many pages from Google.

Rewriting every usage of get_all_pages to do all parsing after each page would require a LOT of work but we can at least save some memory with the way we generate all_pages.

Rather than all_pages being a normal list object we can use the builtin Python [shelve](https://docs.python.org/3/library/shelve.html) library to write the list to disk and save memory.