Fix #432 Add index on maps and attachments table

* Add sequence index on maps and attachments for revs(sequence) on DELETE CASCADE

* The fix was taken from
https://github.com/hermanccw/couchbase-lite-ios/commit/efa70ee9e18986dd62488fa1d7d4ac7b91c185f0
Merge pull request #453 from couchbase/feature/issue_432

Fix #432 Add index on maps and attachments table