minor refactoring

Decoupled multiprocessing code from EnsembleLda class.  This reduces the
length of the class by several hundred lines, making it slightly easier
to understand.

Added _generate_topic_models_worker function to clarify distinction
between single-process and multi-process code.

Fixed flake8 problem (l is an ambiguous variable name)

Adjusted _teardown function (removed i parameter, it's only for logs)

Moved _MAX_RANDOM_STATE to module level