Is async supported in Durable Entity?

**Describe the bug**
await function doesn't work in Durable Entity.

**Investigative information**

- Durable Functions extension version: 2.1.0
- durable-functions npm module version: 1.3.1
- Language (JavaScript/TypeScript) and version: JavaScript
- Node.js version: 10.18.0

**To Reproduce**
```js
const df = require("durable-functions");

module.exports = df.entity(async function(context) {
const currentValue = context.df.getState(() => 0);
switch (context.df.operationName) {
case "add":
await new Promise((f, r) => {
setTimeout(() => {
context.df.setState(currentValue + 1);
f();
}, 1000);
});
break;
case "reset":
context.df.setState(0);
break;
case "get":
context.df.return(currentValue);
break;
}
});

```

**Actual behavior**
State isn't updated when "add" is called.

**Additional context**
https://github.com/Azure/azure-functions-durable-js/blob/bbb0f8b6a2f3fa4b628686f74b196d88edf80a4e/src/entity.ts#L33

Start at src/entity.ts#L33 and run the provided JavaScript Durable Entity reproduction with the listed Durable Functions and Node.js versions. Trace how the async entity handler is invoked and verify whether the add operation updates state after the delayed promise resolves; done means the supported behavior is established and reflected in an appropriate project test or documentation.

Written by the indexing model from the issue text.