fix: Return promise in put

This should probably be covered by the test suite in the future,
however the problem was that when the writing of the changes takes time
micro-analytics will not wait for it to finish. In the tests for
micro-analytics were we mock out flat-file-db and add latency this
breaks several tests.

Author: relekang

index.js

@@ -23,10 +23,10 @@ const observable = new Observable((observer) => {
 
 module.exports = {
   put: (key, value) => {
-    db.put(key, value)
     handlers.forEach(handler => {
       handler({key, value});
     })
+    return db.put(key, value)
   },
   has: (key) => Promise.resolve(db.has(key)),
 	keys: () => Promise.resolve(db.keys()),