(#1738) - single batch() for leveldb bulkDocs()

This modifies leveldb's `bulkDocs()` so that it does
everything in a single batch. Note the addition
of the LevelTransaction to make it easier to
accomplish this.

`leveldb.js` is also refactored to use utils.processDocs,
just the same as idb and websql, cutting down on
code duplication.