Have DumpPool return futures

Have DumpPool return futures when tasks are enqueued, instead of
having users register their task with a string. This change has the
following benefits:

* Simplifies DumpPool since it no longer needs to keep the map of
  strings to futures.

* Allows adding a set of tasks to DumpPool that might not be known at
  build time. For example, we can add a bunch of tasks from a vector,
  without needing to generate arbitrary strings for them.

* Reduces the ability to misuse DumpPool. Before this change, the user
  had to come up with strings representing each task, and needed to
  know a task with said name existed. This change moves it from being a
  runtime error to being a compile time one.

Test: atest dumpstate_smoke_test

Change-Id: Ic6c1a983b049e5236ac596e8af200a21c07ac31b