Moved read timestamp cache and read queue to be per-range. Unfortunately, resetting these data structures is necessary during range leadership change and so keeping them per-store would require additional complexity. Neither data structure is...

Summary: ...particularly large, so should be OK.

Test Plan: go test

Reviewers: bdarnell, andybons

Reviewed By: bdarnell, andybons

Subscribers: andybons, team

Differential Revision: http://phabricator.cockroachdb.org/D117