fix: race condition in getFormat (#299)

There can be a race condition in ipld, if multiple requests to `getFormat` happen concurrently. While one of the requests `await`s `this.loadFormat`, another request might finish loading it, and both end up calling `this.addFormat`.

But if multiple end up calling that function then `this.addFormat` throws an error:

https://github.com/ipld/js-ipld/blob/021b4195b2c1254c323a9f370507fbaa0a42a112/src/index.js#L65-L76

The solution is a basic software transactional memory (STM) idea: If your reads are invalidated during your transaction, retry the transaction.

In its purest form an STM solution would mean:
```js
async getFormat (codec) {
  // TODO vmx 2019-01-24: Once all CIDs support accessing the codec code
  // instead of the name, remove this part
  if (typeof codec === 'string') {
    codec = multicodec.getCodeFromName(codec)
  }

  if (this.resolvers[codec]) {
    return this.resolvers[codec]
  }

  // If not supported, attempt to dynamically load this format
  const format = await this.loadFormat(codec)
  if (!this.resolvers[codec]) { // detect read invalidation
    return getFormat(codec) // retry
  }
  return format
}
```

But we can manually inline the `getFormat` call and slightly improve the if-condition to be more readable and we end up with the code in this PR.

src/index.js

@@ -416,7 +416,11 @@ class IPLDResolver {
 
     // If not supported, attempt to dynamically load this format
     const format = await this.loadFormat(codec)
-    this.addFormat(format)
+    if (this.resolvers[codec] == null) {
+      this.addFormat(format)
+    } else {
+      return this.resolvers[codec]
+    }
     return format
   }
 }