Explore shelve for connected components dict

[NickCrews]

If the connected components dict in clustering.union_find() is running us out of memory, maybe we can use https://docs.python.org/3/library/shelve.html as a disk-based dictionary?

[fgregg]

i think it's worth a look, but when we looked at shelve before for other purposes in the library it was surprisingly slow.

[NickCrews]

Good to know. I'd say reopen if we add profiling as in #967 and see that this actually is a bottleneck.