[power][broker] Use FxHash for performance

* Default hasher is designed to protect against DOS hashing attacks but
  is slow for simple keys. Use FxHash here instead since we control the
  keys.

Change-Id: I47673ff44c436977202a85ca82a9576d4af75ecc
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1551822
Reviewed-by: Onath Dillinger <claridge@google.com>
Commit-Queue: Kyle Gong <kgong@google.com>