New Progress Bar, Backoff, Batching

[soldni]

This PR adds three nice features to BaseParallelProcessor:

• Refactors progress bar out of parallel.py
• Adds a PoolWithDebug wrapper around multiprocessing.Pool that transparently disables multiprocessing when debugging
• Uses backoff library to implement backoff and retries in case of failure
• Ability to create parallel processor that work in batch mode (will change tokenizer processor later to use this new functionality)