As part of UNDERTOW-2429 we are using a new string builder at every new handleEvent invocation.
But, because every byte is copied to the builder, we might end up getting multiple invocations of handleEvent for a single request. This means we could be losing bytes because we are no longer sharing the string builder. To prevent that, we need to share a StringBuilder across multiple invocations, while still preventing data from previous requests to leak inadvertently.