Optimize _clear()

[vweevers]

The clear() method currently uses a JS-land iterator, which is inefficient. We can instead iterate and delete in C++. Rough plan to get there:

• Refactor this to avoid a repeated delete start, by doing the logic in reverse, and remove the need for this by using an std::string for start
• Reuse that code to create an iterator
• Maybe merge the code that does initial seek with code that does manual seek
• Write ClearWorker
• Write NAPI_METHOD(db_clear)

Ref Level/community#79