Fixes a race condition where multiple greenthreads were attempting to resize a file at the same time.  Adds tests to verify that the image caching call will run concurrently for different files, but will block other greenthreads trying to cache the same file.
Fixes a race condition where multiple greenthreads were attempting to resize a file at the same time.  Adds tests to verify that the image caching call will run concurrently for different files, but will block other greenthreads trying to cache the same file.
n0ano/ganttclient