Add own `status` and remove `isXX()` methods

[vweevers]

Follow-up for TODO comments added in #725. Since that PR, the levelup interface has a status property the same as abstract-leveldown. It returns this.db.status.

Ideally levelup has its own status that is tracked separately from the underlying db. That way open events and externally seen status will be consistent. E.g. if the underlying db.status is initially 'open' (rather than 'new'), the levelup status should nevertheless be 'new', and the two states will "consolidate" upon open().

For API parity (Level/community#58) and simplicity, we should also prefer using db.status over these 3 utility methods: isOpen(), _isOpening() and isClosed(). Be aware that subleveldown uses those atm. Keep the isOperational() utility method because that expresses something different: whether operations can be made on the db regardless of its status.

[vweevers]

Closing for the same reasons as #728 (comment).