jsii libraries are not thread-safe

## :rocket: Feature Request

### Affected Languages

- [ ] `TypeScript` or `Javascript`
- [ ] `Python`
- [x] `Java`
- [ ] .NET (`C#`, `F#`, ...)

### General Information
* **JSII Version:** 0.14.2
* **Platform:**

### What is the problem?
```
Exception in thread "Thread-1" software.amazon.jsii.JsiiException: Malformed object reference. Expecting $jsii.byref
at software.amazon.jsii.JsiiObjectRef.parse(JsiiObjectRef.java:52)
at software.amazon.jsii.JsiiClient.createObject(JsiiClient.java:91)
at software.amazon.jsii.JsiiEngine.createNewObject(JsiiEngine.java:458)
```

When trying to run jsiiengine createNewObject in a multithreaded env, it causes above exception.

Start by reproducing the multithreaded createNewObject failure and trace the stack through JsiiEngine.createNewObject, JsiiClient.createObject, and JsiiObjectRef.parse. Determine which shared runtime state causes the malformed object reference, then add a regression test covering concurrent Java calls and verify that the exception no longer occurs.

Written by the indexing model from the issue text.