Address python style issues in MobWrite server

The MobWrite server is written in python, and was adopted by CodeCity. It has numerous python style issues, raised in PR #198, including:

- Extensive use of global variables.
- Generally poor object-orientation / encapsulation.
- Unnecessarily complicated `__init__` signatures.
- Using `try`/`finally` instead of `with` for locking, etc.

Rewrite it conform to [the style guide](https://github.com/google/styleguide/blob/gh-pages/pyguide.md) (or alternatively rewrite it to conform to [the *other* style guide](https://google.github.io/styleguide/jsguide.html)).