Document automatic sequences

[kostja]

• new option in create_index sequence={true|name|id}
• box.schema.sequence.{create/drop/alter}
• box.sequence..nextval()
• access control
• limitations (cross-engine transactions and nextval() don't work, unless the sequence is automatic)

Original implementation is a series of patches tracked in http://github.com/tarantool/tarantool/issues/389