support deletion in mvcc

Summary:
1. encode a prefix in value to indicate if this value is deleted or not.
2. change the engine.go interface to take []byte directly instead of Value since engine don't need to know the checksum and timestamp.
3. implemented del api of mvcc

Test Plan: go test

Reviewers: spencerkimball

Reviewed By: spencerkimball

Subscribers: Tobias, team

Differential Revision: http://phabricator.cockroachdb.org/D108