Add new cache FS, various bug fixes

[JohnStarich]

• Add new cache FS
• indexeddb FS:
  • Bump idb to latest to pick up context leak fixes
  • Preallocate results slice for transaction.Commit()
• keyvalue FS:
  • Fix data race in blob.Bytes when reslicing for a View
  • Fix blob.Bytes data races when sharing byte slices
  • Fix SetBlob arg naming to match behavior
• fstest:
  • Trigger tb.Skip() before sub-tests run to avoid panic(nil) complaints
  • Skip Writer tests for read-only FSs
• mem: Return original blob when retrieving file data