Fix tightlooping when flush()ing without any logs

The promise would resolve immediately, nulling out `flushPromise`. This would
then immediately be set from `new Promise((resolve, reject) => {...})` turning
it back into non-null `flushPromise`. The resolve handler was called so the
next `flush()` would see "oh yes, there is a non-null `flushPromise`" then
promptly try to set `flushAgainPromise` which chains off the resolved
`flushPromise` which relied on `flushPromise` being `null`ed out after
`resolve()`, causing the chained `flush()` to see "oh yes, there is a non-null
`flushPromise`" which... ad infinitum.

This PR fixes it by making the nulling out asynchronous but the fact it took
me this long to debug this issue indicates to me that this is a terrible piece
of code. Will re-write.

src/vector/rageshake.js

@@ -201,13 +201,11 @@ class IndexedDBLogStore {
             if (!this.db) {
                 // not connected yet or user rejected access for us to r/w to
                 // the db.
-                this.flushPromise = null;
                 reject(new Error("No connected database"));
                 return;
             }
             const lines = this.logger.flush();
             if (lines.length === 0) {
-                this.flushPromise = null;
                 resolve();
                 return;
             }
@@ -217,18 +215,20 @@ class IndexedDBLogStore {
             let lastModStore = txn.objectStore("logslastmod");
             lastModStore.put(this._generateLastModifiedTime());
             txn.oncomplete = (event) => {
-                this.flushPromise = null;
                 resolve();
             };
             txn.onerror = (event) => {
                 console.error(
                     "Failed to flush logs : ", event
                 );
-                this.flushPromise = null;
                 reject(
                     new Error("Failed to write logs: " + event.target.errorCode)
                 );
             }
+        }).then(() => {
+            this.flushPromise = null;
+        }, (err) => {
+            this.flushPromise = null;
         });
         return this.flushPromise;
     }