feat: Improve sub-collection watchers.

[amiller-gh]

Currently, sub-collections get their own cache documents. This is good because sub-collections can get pretty large sometimes!

However, for sub-collections of the same type, it would be nice to know exactly which ones are dirty. We can conceivably keep a generation cache document for the sub collection type to know which sub-collections contain a dirty object.

ex:

collection/0/sub-collection

{
  "collection/document/sub-collection/0": 0,
  "collection/document/sub-collection/1": 1,
  "collection/document/sub-collection/2": 0
} 

collection/1/sub-collection

{
  "collection/document/sub-collection/0": 0,
  "collection/document/sub-collection/1": 0,
  "collection/document/sub-collection/2": 0
} 

collection/*/sub-collection

{
  "0": 1,
  "1": 0
} 

We can watch all sub-collections by calling FireStash.watch('collection/*/sub-collection'). This will ensure that all local sub-collections are up to date with minimal reads. A corrosponding event should be fired on any sub-collection change.

On firestash update calls, this will give us another document to bulk-update... but I think this may be worth it to reduce ballooning read requests.