Clear instead of del; clearRange

Summary:
Rename del() to clear() in engine.go
Add clearRange() to engine.go
Test in engine_test.go

Test Plan: go test

Reviewers: spencerkimball, lancevac

Subscribers: team

Differential Revision: http://phabricator.cockroachdb.org/D104
Merge pull request #33 from cockroachdb/manik/del-clear

Clear instead of del; clearRange