Added logic for Store.ExecuteCmd to update local node clock, verify max clock drift, and use the read timestamp cache and read queue appropriately depending on whether the command is read-only or mutating.

Summary: Logic for concurrent readers and writers in store.ExecuteCmd

Test Plan: go test

Reviewers: Tobias, bdarnell

Reviewed By: bdarnell

Subscribers: team

Differential Revision: http://phabricator.cockroachdb.org/D111