consider using standardized MultiCollection if/when it exists

[bluegenes]

#102 introduces a light MultiCollection for storing multiple Collections. The implementation here is very lightweight, and is only used to:

1. load from existing zipfiles (prior batches that succeeded)
2. iterate across these existing sigs to build a Hashmap of BuildParams that can be used to filter out any template signatures that already exist in the batched zips.

If MultiCollection from branchwater (sourmash-bio/sourmash_plugin_branchwater#430) gets standardized and introduced into sourmash core, we could likely use that here instead. However, we would still need to have a buildparams_hashmap implementation.

Related:

• consider how to support more flexible Collection in RevIndex for external storage sourmash#3321