This change removes the incumbent/fetching record concept that allowed
committing a fetching resource to cache and match/matchAll to return a
fully written existing resource, if any, with priority over a fetching
resource. After this change, add/addAll/put promises resolve when the
resources ared fully fetched and committed to cache. match/matchAll will
not return any on-going fetching resources.

This changes the specification type of underlying cache objects from a
map (request to response map) to a list (request response list). The
change is to make the arguments and the return values of the Cache
methods and algorithms (Batch Cache Operations and Query Cache) conform
to one another. The list type seems fine as the algorithms tend to
iterate through the list to find certain items with search options.
Looking at the details of the acutal implementations, I plan to update
it further if needed.

Fixes #884.

The changes include:
 - Replace CacheBatchOperations dictionary which isn't exposed to
   JavaScript surface with cache batch operations struct.
 - Do not store JS objects in the storage but store request and response
   structs instead.
 - Create and return JS objects in the target realm when requested (from
   matchAll() and keys()).
 - Simplify "put" operation related steps by moving/refactoring the
   post-Batch Cache Operation steps, which clear the invalid items, from
   addAll() and put() into Batch Cache Operations.
 - Move the argument validation steps of cache.keys() out of the
   parallel thread to main thread.
 - Fix cacheStorage.keys() to run the steps async. (For now, it still
   runs just in parallel, but later I plan to use the parallel queue
   concept: https://html.spec.whatwg.org/#parallel-queue).

- Adjust variable scope
- Change to early-exit in fetch abort cases
- Fix async steps of fulfillment handlers
- Change the interface of Batch Cache Operations algorithm
 . Change to not return a promise
 . Remove the in parallel steps and make it work synchronously
 . Change the call sites to call it in a created promise's in parallel
   steps