vmpstr@ discovered an implicit copy constructor resulting
in a refcount bump in https://codereview.chromium.org/1895903003/
and made it explicit in https://codereview.chromium.org/1901223003/
this CL removes the need for it via a couple changes:

1) Make SequenceAndSortKey move-only
 - Allows it to be in std::priority_queue without std::unique_ptr's
  - Less pointer chasing == yay?!
 - Removing need for custom SequenceAndSortKeyComparator.
2) Add a take_sequence() method to it:
 - Allows to take its Sequence without refbump on Pop()
3) Make it a private implementation detail and make Transaction's API:
 - Push(sequence,sort_key), PeekSortKey(), PopSequence()
4) Do fdoray's TODO on adding IsEmpty() to Transaction's API:
 - Removes need for null SequenceAndSortKey (required change to allow for PeekSortKey).
5) Make SequenceSortKey copyable.
 - Required for SequenceAndSortKey's move constructor
 - Means it turns into a class with operator== for tests instead of custom
   test member verifiers.

Might have been able to split these up but they all line up fairly nicely
and don't amount to a crazy amount of code so here it is :-).

BUG=553459

Review-Url: https://codereview.chromium.org/1903133003
Cr-Commit-Position: refs/heads/master@{#390451}