Improve TextDataset building/tokenization (6x faster; Enable large dataset file usage)
## This PR:
- Chunks the reading of the dataset file used to create a `TextDataset` for training, if you used a file of any larger size (for my case I had 3.5GB txt file I was able to get finish in ~45 min) the program would just hang at `f.read()` 😢 
- Speeds up `lowercase_text` in the `BasicTokenizer` with a simpler regex scheme
- Add `@functools.lru_cache()` to several functions responsible for acting on individual chars
- Use multiprocessing to drastically speed up tokenization inside `TextDataset` constructor

## Checkouts & Performance Profiling
Benchmark script I used to clock speeds 👉 [gist](https://gist.github.com/mttcnnff/f192d8933f2a8d2b58e14b53841c4080)

Comparison script I used to compare results 👉 [gist](https://gist.github.com/mttcnnff/503f9facd3c317e7efc61a43295a335f)

### Performance on master with no changes on a ~16.5MB txt file (~1 min):
<img width="703" alt="image" src="https://user-images.githubusercontent.com/17532157/70959441-4aa1ae00-204a-11ea-95ad-b1d89baf2e09.png">

### Performance after all changes applied on same ~16.5MB txt file (~10 seconds):
<img width="788" alt="image" src="https://user-images.githubusercontent.com/17532157/70959476-6c9b3080-204a-11ea-9228-6bd55332fff0.png">
@thomwolf or @julien-c any idea what's going on with the seg fault in the `build_py3_torch_and_tf`  [run](https://circleci.com/gh/huggingface/transformers/9811?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link)? was able to run and pass locally...
![image](https://user-images.githubusercontent.com/17532157/71120378-47253880-21aa-11ea-9f62-d0a0fd6e4ff3.png)
  <|||||>> @thomwolf or @julien-c any idea what's going on with the seg fault in the `build_py3_torch_and_tf` [run](https://circleci.com/gh/huggingface/transformers/9811?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link)? was able to run and pass locally...
> ![image](https://user-images.githubusercontent.com/17532157/71120378-47253880-21aa-11ea-9f62-d0a0fd6e4ff3.png)

I've encountered this problem too while adding my PR some days ago. It was broken something inside Transformers library, but @LysandreJik solved this bug two days ago in #2207. Therefore, you can re-launch your tests and you'll see that now it works as expected! :)

UPDATE: you've to install transformers from source code `master` branch through `pip install git+https://github.com/huggingface/transformers.git`<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
