WIP: use async functions from persistence module in datastore

lib/datastore.js

@@ -610,12 +610,7 @@ class Datastore extends EventEmitter {
 
     // Update the datafile
     const updatedDocs = modifications.map(x => x.newDoc)
-    await new Promise((resolve, reject) => {
-      this.persistence.persistNewState(updatedDocs, err => {
-        if (err) return reject(err)
-        else resolve()
-      })
-    })
+    await this.persistence.persistNewStateAsync(updatedDocs)
     if (!options.returnUpdatedDocs) return { numAffected: numReplaced }
     else {
       let updatedDocsDC = []