the query cache now comes from a context, meaning users can easily override and control the cache being used inside their application

this commit does the follwing:
track all active query caches added via the provider, handles mounting/unmounting
if creating a cache, ensure we only do it once
when unmounting an internally-created cache, make sure to clear it first
setFocusHandler now loops through all of the currently-active caches

Co-authored-by: Tanner Linsley <tannerlinsley@gmail.com>