Add opj_codec_set_threads() in public API and propagate resulting thread pool to...

Add opj_codec_set_threads() in public API and propagate resulting thread pool to tcd level

By default, only the main thread is used. If opj_codec_set_threads() is not used,
but the OPJ_NUM_THREADS environment variable is set, its value will be
used to initialize the number of threads. The value can be either an integer
number, or "ALL_CPUS". If OPJ_NUM_THREADS is set and this function is called,
this function will override the behaviour of the environment variable.