[JAVA] Memory leak, stack overflow and incorrect callbacks with okhttp-gson

Description

In Java/libraries/okhttp-gson/api.mustache, the following lines add interceptors that never get removed:

if(progressListener != null) {
  apiClient.getHttpClient().networkInterceptors().add(new com.squareup.okhttp.Interceptor() {
    ...

They pile up until the interceptor chain eventually causes a stack overflow.

Also, different interceptors from different requests get mixed up, making new requests invoke callbacks on previous requests.

Also, this makes the generated class thread-unsafe.

Progress listeners get added whenever an API callback is given, so this doesn't just happen to (the presumably rare) users who implement progress bars.

Swagger-codegen version

The library I'm using was generated with 2.3.0-SNAPSHOT I believe.
The code is unchanged in master.

Steps to reproduce

Make lots of ...Async() calls with a non-null callback parameter on the same api object.

Suggest a fix/enhancement

I don't see a way to add interceptors to individual requests in okhttp, so I guess a single interceptor should be made to look up (using Request.tag perhaps) which callback it needs to call.

Start in Java/libraries/okhttp-gson/api.mustache and inspect how Async calls add network interceptors to the shared HTTP client. Reproduce the issue with many calls using a non-null callback on one API object, then verify that interceptors do not accumulate, callbacks are not mixed between requests, and the generated class is thread-safe.